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ky & Telescope Digital Extra 




WEB EXTRAS Photo Gallery 



SEPT. 2012 ISSUE 

Solar System Storms 
Meteor Shower Maps 



Vesta Close-ups 
September Occultations 



Monthly Audio 
Sky Tour 



THIS WEEK'S 

Sky at a Glance 



Image by 

Marian Lucian Achim 



Shop (a) Sky 



New 

Moon 

Globe 



PBS PROGRAM 

SKY WEEK 

► August 27 - Sept. 2 
Earth, Moon, 
Pluto, and Charon 

► Sept 3-9 

Lyra, the 
celestial Lyre 

► Sept. 10 -16 
The Milky Way's 
Great Rift 

► Sept. 17 - 23 
Close encounter 
with Uranus 

► Sept. 24 -30 
Draco, 

the Dragon 



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SKY 

& TELESCOPE 



September 2012 




VOL. 124, NO. 3 

OBSERVING SEPTEMBER 

43 In This Section 

44 September's Sky at a Glance 

45 Binocular Highlight 

By Gary Seronik 

46 Planetary Almanac 

47 Northern Hemisphere's Sky 

By Fred Schaaf 

48 Sun, Moon & Planets 

By Fred Schaaf 

50 Celestial Calendar 

By Alan MacRohert 

54 Exploring the Solar System 

By Thomas Dobbins 

58 Deep-Sky Wonders 

By Sue French 

63 Going Deep 

By Steve Gottlieb 

S&T TEST REPORT 

38 S&T Test Report 

By Alan Dyer 

ALSO IN THIS ISSUE 

6 Spectrum 

By Robert Naeye 

8 Letters 

10 75, 50 & 25 Years Ago 

By Roger W. Sinnott 

14 News Notes 
66 New Product Showcase 
76 Gallery 

86 Focal Point 

By Dan Rinnan 



SKY& TELESCOPE (ISSN 0037-6604) is published monthly by Sky & Telescope Media, LLC, 90 Sherman St., Cambridge, MA 02140-3264, USA. 
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On the cover: 
Saturn's latest 
Great White 
Spot expanded 
to encircle the 
planet like a 
stormy snake. 

NASA/jPL-CALTECH /SSI 



FEATURES 

New Meteor 
Showers Discovered 

As Earth circles the Sun, it crosses 
a rich, changing tangle of meteoroid 
streams. Help us map them with 
automated video. 
By Peter Jenniskens 

Nature's Wrath 

The solar system's wild weather can 
make Earth's extremes seem serene. 
By David Baker & Todd Ratcliff 

Protoplanet Close- Up 

Dawn is revealing the history of 
Vesta, a unique world that is part 
asteroid and part planet. 
By Jim Bell 

Finding the Limit 

for Uneven Double Stars 

Your observations can contribute 
to this important project. 
By Sissy Haas 

Easy Reflector Collimation 

Achieving precise optical alignment 
is neither difficult nor time-consum- 
ing — provided you stay on course. 
By Gary Seronik 




There's more to find online @ 

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SKY AT A GLANCE 

Peruse the next week's celestial 
delights in our popular column. 
Also available as an app! 
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SKYWEEK 

Watch full episodes of our 
weekly PBS segment on what's 
currently up in the sky. 
skyweek.com 

NEW EQUIPMENT DIRECTORY 

Browse hot products for all 
types of observing. 
SkyandTelescope.com/directory 

TIPS FOR BEGINNERS 

New to astronomy? Here's 
everything you need to jump 
into the fun. 
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Robert Naeye 
Spectrum 




A Great Transit Experience 




W^") the S&T Transit of Venus trip to the Big Island of Hawaii was a fantas- 
tic experience. Our tour group enjoyed beautiful views of the transit, espe- 
cially during the critical periods of ingress and egress. Everyone felt exhila- 
ration after having observed one of nature's rarest spectacles under clear 
Hawaiian skies and comfortable temperatures. 

Our group consisted of about 120 people, many of whom were S&T sub- 
scribers or their spouses. We had folks from all over the U.S. and from Aus- 
tralia, Canada, Colombia, Hong Kong, Mexico, and the U.K. As usual on such 
trips, the participants had an eclectic range of interests and backgrounds, 
which made for a lot of interesting conversations. 

On transit morning, vans transported some of the people in our group to 
the 13,796-foot summit of Mauna Kea, where they enjoyed tours of the Subaru 
Telescope before and during the transit. The rest of us set up our gear at the 
Onizuka visitor center, at an elevation of 9,300 feet. During the 6V2-hour tran- 
sit, various other people went to the summit for tours of Subaru. I had visited 
the summit in 1999 and 2007, so I decided to stay put at the visitor center. 

A western ridge blocked the final hour of the transit from the visitor cen- 
ter, so most people in our group rode back to our beachfront hotel in Waiko- 
loa and watched egress over the Pacific. The rest stayed behind and watched 
the finale from the summit. My only disappointment for the day was that the 
Sun set in a distant fog bank, ruining any chance of seeing a green flash. 

I found it interesting that observers using hydrogen-alpha filters (includ- 
ing me) saw first contact about a minute before observers using standard 
white-light filters did; we also saw fourth contact about a minute after the 
white-light observers saw it. These bonus 2 minutes were due to the fact that 
H-alpha scopes show the chromosphere, which is above the photosphere. 

I want to thank Neil Bauman and Theresa Mazich of Insight Cruises 
for an extremely well-organized trip, along with our guides: Charlie Datin, 
Aaron Melendrez, and Curt Duffy. A special thanks goes out to Wayne 
Fukunaga and Star Gaze Hawaii for providing 18 additional scopes and 




continual weather updates. 



Editor in Chief 



Founded in 1941 

by Charles A. Federer, jr. 

and Helen Spence Federer 

The Essential Magazine 
of Astronomy 

EDITORIAL 

Editor in Chief Robert Naeye 

Senior Editors Dennis di Cicco, Alan M. MacRobert 

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Editor Emeritus Richard Tresch Fienberg 

Senior Contributing Editors J. Kelly Beatty, Roger W. Sinnott 

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David Grinspoon, Paul J. Heafner, Ken Hewitt-White, Johnny Home, E. C. 
Krupp, Emily Lakdawalla, David H. Levy, Jonathan McDowell, Rod Mollise, 
Donald W Olson, Fred Schaaf, Govert Schilling, Gary Seronik, William Sheehan, 
Mike Simmons, Alan Whitman, Charles A. Wood, Robert Zimmerman 

Contributing Photographers P. K. Chen, Akira Fujii, Robert Gendler, 
Babak Tafreshi 

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Design Director Patricia Gillis-Coppola 
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-& Letters 



Write to Letters to the Editor, Sky & Telescope, 

90 Sherman St., Cambridge, MA 02140-3264, 

or send e-mail to letters@SkyandTelescope.com. 

Please limit your comments to 250 words. 



Come and See 

Thank you for Thomas Watson's Focal 
Point, "With My Own Eyes" (May issue, 
page 86). As a Solar System Ambassador 
with NASA's Jet Propulsion Laboratory 
and an amateur astronomer, I hear some 
of the same comments against spending 
time observing: the professionals have 
been doing it for years, so why bother? Yet 
there's nothing like the thrill of locating 
objects in the night sky, and as Watson 
argued, observing fulfills the need to see 
these wonders for myself. That is all the 
meaning I need. For several years now I 
have attended astronomy camps on Mount 
Lemmon near Tucson, sponsored by the 
University of Arizona Alumni Association. 
Each time is as "meaningful" as the last. 

Janet S. Howard 

Memphis, Tennessee 

Watson's remarks touch upon a vital, joy- 
ful, and universal human experience, one 
to celebrate and to encourage. There is no 
substitute for seeing for ourselves. 

Last summer I used a pair of 28x110 
binoculars to show my nephews and their 
wives their first-ever sight of M6 and M7. 
I have yet to see many deep-sky objects, 
so at this stage I still do astronomy 
"backwards" — I go out and look without 
knowing what I am seeking and find out 
what the objects are afterwards. This was 
true for me when I stumbled across M35, 
M36, M37, and M38 at the Okie-Tex Star 
Party at Black Mesa, Oklahoma, a little 
after 5:00 a.m. one year. As Jimi Hendrix 
sang, "'Scuse me while I kiss the sky!" 

When our first granddaughter is old 
enough, I'm going to show her M41. 
Clyde Glandon 
Tulsa, Oklahoma 

The Dark Sky Frontier, Reprise 

I thoroughly enjoyed Tyler Nordgren's 
article "Astronomy in National Parks" 
(May issue, page 26). Not only is it well- 
written, the accompanying photos are 
breathtaking. 



The national parks are indeed at the 
frontier of popularizing observational 
astronomy and raising awareness of 
the problem of (and solutions to) light 
pollution. I volunteer each September 
for the astronomy program at Acadia 
National Park in Maine, which Nordgren 
mentioned. As part of its Annual Night 
Sky Festival, the park throws a major star 
party on Cadillac Mountain. The party 
was rained out in 2011, but in 2010 it was 
attended by more than 1,000 park visitors 
and locals, a jump from the previous 
year's 300 or so. These numbers are only 
a small indication of the increased public 
awareness we are experiencing thanks to 
the untiring efforts of folks like Nordgren 
and the dedicated rangers in the National 
Park Service. I hope S&T will continue to 
keep these efforts in the public eye. 
Bert Probst 
Bay Village, Ohio 




I loved the piece on national parks and 
the emphasis it places on preserving dark 
skies. Sadly, it's the overnight camping 
that keeps many of us away. Those of us 
living within a few hours' drive of Zion 
National Park had a rare chance in the 
last few months, though: Utah Highway 
14 washed out last fall, and since then the 
National Park Service has left the park 
gate open at night to allow area residents 
easy access to Hwy 89. Hwy 14 opened on 
a limited basis at May's end, and construc- 
tion should be complete this month, so 
our chance to see Zion's skies probably 
won't last much longer. Last time I drove 
through, the night skies were spectacular, 
and we didn't see a single car from one 
side of the park to the other. 

Jason Couchman 

Henderson, Nevada 

I enjoyed your national parks article, but 
it missed many of the beautiful parks in 
the eastern United States. I am a member 
of the Smoky Mountain Astronomical 
Society, which is invited each year by the 
National Park Service to conduct a star 
party in the Cades Cove region of the 
Great Smoky Mountains National Park 
on the Tennessee-North Carolina border. 
Cades Cove is one of the most beautiful 
sites in the park, especially at night. The 
first star party was in 2005 and attracted 
approximately 120 attendees; last October's 
gathering attracted more than 850 people. 
We distribute our old S&T magazines at 
the event to anyone who wants them, and 
we think we have added to our society's 
membership as a result. Thank you for 
such a wonderful magazine. 

Duane Dunlap 

Knoxville, Tennessee 

Nordgren's article left out Rock Creek 
Park in Washington, D.C., which is, ironi- 
cally, the only park in the National Park 
Service to have a planetarium. (It's part of 
the Nature Center.) I was the planetarium 
director there from October 2006 through 



8 September 2012 SKY &. TELESCOPE 



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| Letters 

June 2009. The planetarium presents 
programs for the general public, as well as 
for school classes. For the last 50 years or 
so, RCP has collaborated with the National 
Capital Astronomers (of which I am cur- 
rently a trustee) to present an "Exploring 
the Sky" event one Saturday evening each 
month from April through November. A 
few of us bring telescopes and we set them 
up for public viewing. Even though the 
sky is heavily polluted by lights from the 
District and neighboring Maryland cities, 
we usually get around 60 visitors. 

Andrew W. Seacord, III 

Bowie, Maryland 

Call for Telescope Donations 

Astrosphere New Media has started a tele- 
scope donation program in a partnership 
with Galileoscope that I'd like to bring to 
your readers' attention. Telescopes4Teach- 



75, 50 & 25 Years Ago 



September 1937 

The Real Corona "The 
first definite proof that the 
solar corona is not made 
up chiefly of flaming coro- 
nal streamers alone, as 
has been supposed, but is 
an even, globular blanket 
covering the sun more 
than a million miles deep, was revealed [August 
12] by a conference of astronomers at Harvard 
Observatory. 

"Photographs leading to this finding were 
taken from an airplane in the substratosphere 
off the coast of Peru during the eclipse of last 
June 8." 

Astronomers had expected airborne equip- 
ment to reveal more delicate tracery in the corona 
than is seen from the ground. Instead, it made the 
general glow stand out. 

September 1962 

One Odd Comet "The 
general rule that comets 
far from the sun are faint, 
quiescent, and nearly 
structureless objects has 
been dramatically violated 
by Comet Humason 
(1961e). . . . 

"Sudden and striking changes in appearance, 





ers allows individuals and organizations 
to make tax- deductible donations and 
designate a school or teacher to whom 
Galileoscopes will be donated. We are very 
excited about this program, and hope that 
Astrosphere can meet its goal of putting 
5,000 telescopes into classrooms by the 
time school starts in the fall. You can find 
out more at www.telescopes4teachers.org. 

Douglas Arion 

Galileoscope LLC 

For the Record 

# In the bottom zoom image on page 51 of 
thefuly issue, R Draconis is the faint dot 1.5 
mm below and left of the star indicated. The 
top image on the same page is correct. 

# In the August issue, pages 37 and 41, 
images credited to David Makepeace should 
have included Lukas Gornisiewicz in the credit 
or as the sole credit. See skypub.com/errata. 



Roger W. Sinnott 



often in less than a day, have added to the great 
interest Comet Humason has aroused among 
professional astronomers. To record these 
alterations, the 40-inch reflector of the U.S. Naval 
Observatory is especially effective. Elizabeth 
Roemer writes ... 'I think the corkscrew feature 
in the tail on July 7th is real.'" 

Then shining at 8th magnitude from well 
outside Mars's orbit, this peculiar comet remains 
in a class of its own. 



-Sky & 

Telescope. 



September 1987 

Still No Planet X "The 
twin Pioneer 10 and 11 
spacecraft have found 
no evidence for a planet 
beyond the orbit of Pluto. 
. . . [PJrecise tracking 
data indicate that neither 
probe has felt the gravi- 
tational pull of an unseen body while racing 
through the outer solar system. . . . 

"[Apparently reliable 19th-century observa- 
tions of Uranus and Neptune show that their 
orbits were slightly erratic. . . . Pluto and its 
moon aren't massive enough to have shifted 
Uranus and Neptune by the amounts seen. But if 
a larger planet [than Pluto] were responsible, why 
didn't it affect the Pioneers? 

Those 19th-century observations weren't so 
reliable: subtle faws mimicked a Planet X's effect. 



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News Notes 



SCOPES I Closure Nears for Two Hawaiian Observatories 




Faced with too many telescopes and 
too little funding, officials at the United 
Kingdom's Science and Technology Facili- 
ties Council announced they will soon 
close two of the oldest observatories atop 
Mauna Kea. Unless another organization 
takes over operations, the United King- 
dom Infrared Telescope (UKIRT) will shut 
down in September 2013, and the James 
Clerk Maxwell Telescope will close the 
following year. 

It's becoming an all-too-common 
scenario in an era of shifting priorities 
and diminished funding for scientific 



endeavors. The U.K. will still participate 
in several megaprojects, including the 
Atacama Large Millimeter/submillimeter 
Array (ALMA) and the proposed Euro- 
pean Extremely Large Telescope, both in 
Chile. But those buy-ins leave little cash to 
maintain the country's older but still-pro- 
ductive facilities in Hawaii and the Canary 
Islands. (The STFC decision spares the 
U.K.'s three Canary scopes.) 

The news wasn't unexpected: STFC 
signaled its intentions in late 2009. But it's 
still a sad turn of events. UKIRT's board 
claims that the scope's early closure will 



bring an abrupt end to an ongoing deep- 
infrared survey that could have continued 
for an additional year using a budget of 
only about $150,000. 

With an aperture of 12.5 feet (3.8 
meters), UKIRT ranks as the world's 
second-largest telescope dedicated solely 
to infrared observations (surpassed only 
by the U.K.'s recently built VISTA tele- 
scope in Chile). The James Clerk Maxwell 
Telescope has a primary mirror 49 feet 
(15 meters) across, making it the largest 
telescope optimized for observing at sub- 
millimeter wavelengths. Built by the U.K., 
Canada, and the Netherlands, JCMT saw 
first light in 1987; UKIRT, in 1979. 

What's transpiring in the U.K. could be 
a harbinger of equally unwelcome events 
on this side of the Atlantic. A panel of U.S. 
astronomers is wrapping up a "portfolio 
review" for the National Science Foun- 
dation, and when the group releases its 
findings (slated, as we went to press, for 
late this summer), many suspect legend- 
ary facilities such as Kitt Peak National 
Observatory might get the ax. 

Perhaps the novel arrangement NASA 
has struck regarding the Galaxy Evolu- 
tion Explorer (GALEX) mission will 
become more common. NASA has turned 
both the spacecraft's control and funding 
over to a private institution — namely 
Caltech. The transfer, a first for NASA, 
is permitted by the Stevenson-Wydler 
Technology Innovation Act of 1980, 
which encourages federal agencies to 
transfer no-longer-needed resources to 
educational institutions and nonprofit 
organizations. Caltech plans to fund the 
9-year-old spacecraft through domestic 
and international partnerships. So far, 
the institution only has a few months' 
worth of costs spoken for, but the num- 
ber of partners will hopefully go up. 
■ j. KELLY BEATTY 



To get astronomy news as it breaks, 
visit skypub.com/newsblog. 



14 September 2012 SKY & TELESCOPE 



EXOPLANETS I Metal-Poor Stars Parent Small Planets, Too 



Astronomers have found that planets of 
less than four Earth radii can form around 
stars with a wide range of heavy- element 
abundances — even those stars with as 
little as one-fourth the Sun's "metal" con- 
tent. (In astronomy, "metals" are elements 
heavier than hydrogen and helium). 

The results, announced by Lars Buch- 
have (University of Copenhagen) and his 
colleagues at the American Astronomical 
Society's summer meeting and published 
in Nature, appear to run counter to two 
decades of ground-based observations that 
suggest metal-rich stars are more likely to 
harbor planets, particularly gas giants like 
Jupiter. But the new study doesn't actually 
contradict those findings: although the 
team found small planets circle a variety of 
stars (with an average metal content close 
to the Sun's), stars hosting gas giants still 
had metallicities on average 40% higher 
than the solar value. 

Metals are mostly created inside stars. 
Because planets pull their raw material 
from the same source as their parent 




stars, astronomers think the earliest stars 
(which were metal poor) probably didn't 
host planets. Instead, planets arose later as 
the stellar population matured and built 
up stores of carbon, silicon, and oxygen — 
elements common in rocky planets. 

But if small planets don't require high 
metal content, planets may have started 
forming early in galactic history. That 
could mean more planets in general, but 
also more worlds that have had time for 
complex life to evolve. 



Most of the 226 planet candidates in 
the study, all found by NASA's Kepler mis- 
sion, orbit within 0.5 a.u. of their stars. For 
many stars, planets with such tight orbits 
are probably not within the habitable zone. 

"We can now look at basically any star, 
and there's a chance it will have a small 
planet," says coauthor Guillermo Torres 
(Harvard- Smithsonian Center for Astro- 
physics). "This really opens up the possi- 
bility for searches for habitable planets." 

■ STEPHEN P. CRAFT 



NASA I Astronomers' New Spy Scopes 



It turns out chanty isn't dead in 
the government's bureaucratic 
hallways. Astronomers recently 
announced that a federal intel- 
ligence agency has given NASA 
a pair of declassified space tele- 
scopes, at no charge. But like a 
car won on a game show, this gift 
isn't exactly free. 

Unofficially dubbed NRO-1 
and NRO-2 (they belonged to the 
National Reconnaissance Office, 
but they never made it beyond 
the warehouse), the telescopes 
represent a unique opportunity 
to advance science with minimal 
impact on NASA's beleaguered 
bottom line. 

Launching both NRO scopes 
into space is currently beyond 
any NASA accountant's dreams. 
But using one scope could 
help move forward the Wide- 



Field Infrared Survey Telescope 
(WFIRST) project, a mission at 
the top of astronomers' decadal 
wish list (S&T: March 2011, page 
22). WFIRST was envisioned to 
address several high-priority sci- 
ence programs, including inves- 
tigating dark energy and hunting 
for exoplanets. No instrument 
has been built yet, but the project 
was pitched as a 1.5-meter wide- 
field telescope. 

That's a shoe NRO-1 could eas- 
ily step into. Though designed for 
terrestrial viewing, both scopes 
are "optically perfect," says Alan 
Dressier (Carnegie Observato- 
ries), who briefed politicians in 
June on potential uses for the 
scope. Each scope sports a VSoth- 
wave, 2.4-meter (7.9-foot) pri- 
mary mirror, with a field of view 
about 100 times larger than that 



of the Hubble Space Telescope. 
Depending on the doohickeys 
NASA adds, a repurposed NRO 
could have a field of view about 
the same as WFIRST's but could 
see objects twice as faint. 

That capability would make it a 
good partner for the oft-delayed 
James Webb Space Telescope. 
With a larger field of view but 
lower sensitivity, NRO-1 could 
pick out interesting infrared tar- 
gets through surveys that JWST 
could then home in on. 

But NRO-1 is far from ready 
to launch. The telescope is basi- 
cally optics and housing, without 
instruments. It also needs a body 
and a ride to space. 

With the Space Shuttle Program 
terminated, the ride might be the 
biggest problem. But NRO-1 is 
lighter than Hubble and thus less 



expensive to launch. Dressier 
speculates that tests of NASA's 
Space Launch System might pro- 
vide a serendipitous way to lift 
NRO-1 into orbit; private-sector 
launch vehicles such as SpaceX's 
Falcon 9 could work as well. 

Dressier estimates each of 
these obstacles will cost $100 to 
$200 million to overcome. Still, 
the optics and housing — which 
NASA now has — make up the 
bulk of telescope-building costs. 
NRO-1 might be put into service 
for $1 billion, an amount roughly 
one-third Hubble's pre-launch 
construction bill in 2012 dollars. 

Still, NASA's astrophysics bud- 
get is tied up in the (currently) 
$9-billion JWST. Assuming JWST 
deploys by 2018, NRO-1 could be 
put into service by 2020 to 2022. 



STEPHEN P. CRAFT 



News Notes 



INTERSTELLAR SPACE 

No Bow Shock at 
Solar System Boundary 

Measurements from NASA's Interstellar 
Boundary Explorer (IBEX) suggest that the 
Sun doesn't fly fast enough through space 
to create a bow shock in the thin gas floating 
between the stars. This bow shock has been a 
long-standing pillar of scientists' understand- 
ing of the solar system, and the new result 
may overturn decades of research. 

Technically, the solar system's edge lies 
where the solar wind slams into interstellar 
gas. At this point the heliosphere, the vast 
magnetic bubble created by the solar wind, 
hits the (also magnetized) interstellar medium. 
Because the interstellar medium's magnetic 
fields are aligned differently than those of the 
heliosphere, the meeting of solar wind and 
interstellar particles is like oil and water: a 
boundary appears. 

Scientists thought the Sun flew through 
space like a supersonic jet, creating a bow 
shock as the heliosphere plowed through the 
interstellar medium, much like the air pileup 
in front of a fighter plane. But an international 
team reported in the June 8th Science that 
the Sun is moving through this gas at 52,000 
miles (84,000 km) per hour, about 12% slower 
than the speed previously measured by the 
Ulysses spacecraft. 

That slower speed makes the Sun look less 
like a fighter jet and more like a cruising boat. 
IBEX's observations confirm that the Sun is 
still moving through the Local Interstellar 
Cloud, a fluff of gas roughly 30 light-years 
across. But combined with the effect of the 
interstellar medium's relatively strong mag- 
netic field, the Sun's slower advance makes at 
best a bow wave, a region of slightly increased 
density like that around a chugging tugboat. 

"It's too early to say exactly what these 
new data mean for our heliosphere," says 
IBEX principal investigator David McComas 
(Southwest Research Institute, San Antonio). 
"Decades of research have explored scenarios 
that included a bow shock. That research now 
has to be redone." 
_ MONICA YOUNG 




MARS I Dunes on the Move 



Scientists studying images from 
NASA's Mars Reconnaissance Orbiter 
have caught dunes migrating across the 
Red Planet's surface. Common wisdom 
dictates that winds in the planet's thin 
atmosphere can't deliver the strong, sus- 
tained push needed to get sand moving 
on a large scale, but the new observa- 
tions add to growing evidence that this 
assumption is wrong. 

The team used two pairs of snap- 
shots from the orbiter's High Resolution 
Imaging Science Experiment (HiRISE) 
camera to track dune migration in the 
equatorial Nili Patera dune field. The 
measurements, which were taken 105 
Earth days apart, showed that dunes 
moved with an average speed of 0.1 
meter per Earth year. Although that 
migration rate is one-tenth to one-hun- 
dredth the speed of terrestrial dunes 
with comparable heights, the average 
rate of sand transport — the volume of 
sand moved over a certain distance in a 
certain time — is about equal to that of 
dunes in Victoria Valley, Antarctica, the 
team reported in the May 17th Nature. 

The result shows that Martian winds 
can easily send sand moving across the 
surface. What matters is not how hard it 
is for winds to pick up individual grains, 
but what happens when the dancing 
grains fall back down. Sand grains can 
fly higher and longer in Mars's low sur- 
face gravity, gaining more speed than 



Ripples on dunes in Mars's Nili Patera region 
(shown here) moved as much as several 
meters over 105 Earth days. The ripples' 
movement allowed researchers to calculate the 
dunes' migration rate, proving Martian winds 
can in fact move sand on a large scale. 

they would have otherwise. When they 
come bouncing down again, they smack 
into other grains and get them mov- 
ing, too. Mars's low gravity makes this 
easier to do, explains Candice Hansen 
(Planetary Science Institute), who last 
year reported evidence for dune changes 
in the north polar region but was not 
involved with the current study. 

Ultimately, after a few gusts kick 
up some grains, moderate winds could 
keep the dunes moving across the 
landscape. The result implies that Nili 
Patera could have formed in less than 
10,000 years, an interpretation contrary 
to the long-standing idea that Mars's 
dune fields formed in a previous climate 
with a thicker atmosphere. 

The new explanation for sand's 
migration could partially explain the 
existence of global- scale dust storms 
(page 26), explains physicist Jasper 
Kok (Cornell University). Data from 
NASA's Mars Exploration Rovers also 
suggest that low-density clumps of dust 
are easier to lift than sand, but the full 
answer remains elusive, he says. 
■ CAMILLE M. CARLISLE 



16 September 2012 SKY & TELESCOPE 



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News Notes 



PRO-AM I Millions of Stars and the Promise of More 



A pro-am collaboration by the American 
Association of Variable Star Observers is 
producing a detailed photometric all- sky 
map that will fill a sizable hole in cur- 
rent data, AAVSO director Arne Henden 
announced at the American Astronomical 
Society's summer meeting. 

The AAVSO Photometric All- Sky 
Survey (APASS) uses five filters to observe 
stars between 10th and 17th magnitude 
in both hemispheres (S&T: June 2011, 
page 34). This magnitude range has never 
before been systematically surveyed, 
although it partly overlaps previous all- sky 
work in photometry and astrometry, the 
latter including obser-vations by the U.S. 
Naval Observatory's UCAC program. The 
pro-am team also chose these magnitudes 
because they are ones typically observed 
in backyard telescopes. 

Observers will likely still need to rely 
on photometric data taken with bigger 
telescopes for the most crowded parts of 
the sky, says Brian Skiff (Lowell Observa- 



tory), who has spent several years com- 
piling his own database of high-quality 
photometry. "But even within its limits, 
[APASS] should be a boon for observers — 
both professional and amateur — doing 
systematic observing of just about any 
sort," he says. 

Having standardized, high-precision 
photometry for these stars will enable 
astronomers to more accurately (and 
more quickly) determine magnitudes for 
variable stars or for new objects such as 
supernovae. With 42 million stars covered 
by APASS, observers won't have to spend 
as much time searching the sky or their 
images for a comparison star. "Folks have 
wanted a uniform survey like this for a 
long time," says Skiff. 

APASS uses two pairs of 8-inch astro- 
graphs, one in New Mexico and the other 
in Chile. Each pair sits on a Paramount 
ME mount and is operated remotely. One 
scope takes the bluer Johnson B and Sloan 
g' exposures, and the other snaps the red- 



der Johnson V and Sloan r' and i shots. 
Each image is 2.9° x 2.9° in size; so far, 
the survey has covered roughly 95% of the 
sky twice. 

APASS uses solely commercial hard- 
ware and software, working largely thanks 
to vendor support and private grants. 

Henden says APASS 's photometric pre- 
cision is currently about 3%. He expects 
that to improve to 1 to 2% by the final 
release of the catalog in 2014. 

Also on the pro-am scene, a promising 
new program is Lowell Observatory's Low- 
ell Amateur Research Initiative (LARI), 
announced at this year's Symposium 
on Telescope Science in Big Bear Lake, 
California. LARI seeks to take advantage 
of amateurs' enthusiasm and skills by 
assigning applicants to projects suited to 
their interests and capabilities. You can 
find a list of projects currently open to 
collaborators on LARI's site at http://bit.ly/ 
K2z9Wo. 
■ CAMILLE M.CARLISLE 



GALACTIC I M31-Milky Way Crash No Fender-Bender 



New work using Hubble Space Telescope 
observations of the Andromeda Galaxy has 
found that the galaxy will possibly collide 
head-on with the Milky Way in about 4 bil- 
lion years. The conclusion, reported in three 
papers in the July 1st Astrophysical Journal, 
used detailed measurements of M31's proper 
motion to pin down the galaxies' possible 
fates. Uncertainty in previous proper motion 
measurements had left astronomers dubious 
about the exact nature of the expected pile- 



up. Now, the precise HST measurements, 
coupled with careful computer simulations, 
suggest that M31 and the Milky Way have a 
41% chance of striking head-on (defined as 
a collision in which their centers are at most 
82,000 light-years apart). In that case, the 
smaller galaxy M33, which currently orbits 
Andromeda, will settle into an orbit around 
the M31-Milky Way pair. 

However, the authors add that there's a 
small chance (9%) that M33 will strike the 



Milky Way first — or it might get flung from 
the Local Group altogether (7%). The collision 
will likely push the solar system farther out 
from the newly-formed galactic center than its 
current location in the Milky Way. 

Based on the computer simulations, this 
image sequence shows what might happen as 
M31 and the Milky Way collide. The view is set 
25,000 light-years from the Milky Way's center, 
roughly Earth's present galactic address. ♦ 
■ j. KELLY BEATTY 





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Mapping Meteoroid Orbits 



Meteor Showers 
Discovered 




UAN CARLOS CASADO 



20 September 2012 SKY & TELESCOPE 




As Earth circles the Sun, it crosses a rich and changing 
tangle of meteoroid streams. Automated cameras are 
mapping them better than ever. 
PETER JENNISKENS 



FOR THE FIRST TIME in nearly two centuries of sur- 
veying the night sky, astronomers are charting meteor 
showers in a massive and objective way — by automated 
surveillance video. Generations of naked-eye observers 
have discovered and monitored meteor showers by visual 
counting and hand-plotting. Photography and radar have 
helped, but all these methods have their limits. Now we're 
using networks of low-light video cameras and sophisti- 
cated data processing to map meteoroid streams much 
better than ever before. We're finding dozens of new 
minor showers and some surprising not-so-minor ones. 
Amateurs are participating, and we're seeking more help 
in this ongoing effort. 

Until now, astronomers have agreed on only a short list 
of 64 "established" meteor showers. At the August Interna- 
tional Astronomical Union's General Assembly in Beijing, 
which will take place about two months after this issue 
goes to press, we expect to expand this official list greatly. 

These developments are quite recent. Our Cameras for 
Allsky Meteor Surveillance (CAMS) project, first came 
online in October 2010. In just over a year it has identified 
new showers and confirmed many previously question- 
able ones that we hope will be approved in Beijing. 

Tiny Bits Betray Big Parents 

Each meteor shower is important, no matter how minor 
it appears. A shower happens when Earth, in its annual 
orbit around the Sun, passes through a sparse stream 
of sesame-seed to pebble-sized dust clods and bits of 
rock that have been shed by a mountain-sized comet or 
occasionally by an asteroid. This parent body follows an 
orbit similar to its debris stream, which means that even 
a minor shower can betray the presence of a comet that 
might someday hit Earth. Meteor showers indicate these 
potentially hazardous objects even if the parent has yet to 
be discovered. Showers also provide clues to what these 
objects are made of and how they have behaved in the 
recent past, how they shed particles when warmed by the 
Sun and, in many cases, at what times they have under- 
gone massive disruptions. 

If we can identify the source body, we can calculate 
the shape and size of the stream in space by determining 
how the parent was moving when each wave of shed- 
ding occurred, and how each swarm from these episodes 
has been gravitationally perturbed since. The current 



searches for near- Earth objects have been identifying the 
parent bodies of numerous showers for the first time. 

Over many millennia, a meteoroid stream gradually 
spreads out and falls apart, creating the great background 
of random sporadic meteoroids that impact spacecraft 
unpredictably and give rise to the solar system's vast 
zodiacal dust cloud. 

Mapping the Meteor Year 

To my surprise, the first week of December has turned 
out to be the most interesting for meteor observations. 
The new CAMS data show that at least 14 significant 
showers are active that week, each with its own particular 
radiant direction on the sky, velocity, duration (indicating 
stream width), and behavior. Five of these showers were 
completely new to me, even after spending many nights 
over the years on my back in a reclining lawn chair, pen 
and paper in hand, watching the sky intently. 

That's how meteor showers used to be discovered. 
Since about the 1860s, visual observers have mapped 
them by drawing meteor paths on a star chart and then 
looking for a divergence point: the shower's radiant, 
its perspective point of origin on the sky. The radiant 
is where we would see a shower's meteoroids coming 
from if we could see them in the far distance, instead of 
just in their final second or two when they dive into the 
atmosphere. The particles in a stream move essentially 
in parallel — so like railroad tracks in a photograph, they 
appear to diverge from a distant perspective point. 

Starting in the 1930s, astronomers have also identi- 
fied showers not just by meteors' two-dimensional 
paths on the sky, but by their actual three-dimensional 
paths through Earth's atmosphere as triangulated from 
sky cameras positioned some 25 to 60 miles (40 to 100 
km) apart. Working backward from this path through 
the atmosphere, we can determine a meteoroid's prior 
orbit through the solar system. But photographic film 
and plates were relatively insensitive, and such efforts 
produced few orbits. A great many showers were reported 
based on suggestive, but statistically weak, similarities 
among a mere two or three such calculated orbits. 

The list of showers grew even longer in the 1940s, 
when radar was first used to count meteors and mea- 
sure rough trajectories in the atmosphere. This was how 
astronomers mapped daytime showers. By that time, 



SkyandTelescope.com September 2012 21 



Mapping Meteoroid Orbits 



however, meteor astronomers had often given up on checking 
their results against those of other researchers, and soon many 
showers were known by multiple names. Only in 2006 did the 
International Astronomical Union resolve this confusion by 
adopting nomenclature rules and establishing a Task Group on 
Meteor Shower Nomenclature, which I chair. 

We name officially accepted showers for the constellation 
containing the shower's radiant, or the bright star nearest to 
it. The Latin possessive ending of the star name is replaced by 
"-id." For example, when Earth passes through Comet Halley's 
meteoroids in early May, we see them radiating from near the 
star Eta Aquarii, so they are called the Eta Aquariids (pro- 
nounced Aquari-ids). In case of confusion, the task group can 
propose a preference. Names are only official once a stream is 
convincingly established as real. 



The Meteor Data Center in Poznan, Poland, maintains a 
Working List of Meteor Showers, which helps us keep track of 
known and suspected showers mentioned in recent literature. 
New shower discoveries need to be first verified against that list 
and are then assigned a unique name and three-letter code. 

This working list was 385 showers long as of June. At the 
IAU General Assembly in Rio de Janeiro in 2009, 64 of them 
received official names and the designation "established." But 
that still left more than 300 showers awaiting confirmation. 

The CAMS Project 

CAMS grew out of my frustration in trying to construct a list of 
known meteor showers for my 2006 book Meteor Showers and 
their Parent Comets. Based on the literature, it was impossible to 
tell which ones were real beyond a handful of main showers. 




Since mid-2008, NASA's planetary-astronomy program 
has supported the CAMS project to confirm the unconfirmed 
showers. It took several years to develop the necessary technolo- 
gies to deal with data from 60 video cameras divided over three 
stations. First light was in October 2010, and all stations were 
working by May 2011. 

We analyze the data at the SETI Institute, my home institu- 
tion near NASA's Ames Research Center. A team of 10 amateur 
astronomers support the data analysis and operate our video 
stations, located around the San Francisco Bay area. The first 
station is at the Fremont Peak Observatory south of Gilroy, Cal- 
ifornia, and is run by amateurs of the Fremont Peak Observa- 
tory Association (FPOA). A second station is at a private home 
in Sunnyvale, currently with Jim Albers. The third is at Lick 
Observatory, operated by staff astronomer Bryant Grigsby. 

How do we do it? Each station has 20 Watec Wat-902H2 Ulti- 
mate video security cameras aimed around the sky. These can 
see stars as faint as magnitude 5.4 when equipped with 12-mm 
objective lenses. Each camera watches a 20°-by-30° area, allow- 
ing 20 cameras to cover the entire sky above 30° elevation. 

The heart of the project is the software needed to detect 
meteors in video recordings, triangulate on them from differ- 



ent stations, and calculate their 3-D trajectories in the atmo- 
sphere and prior orbits in space. The main architect of this 
software is amateur astronomer and professional detection- 
algorithm specialist Pete Gural of Sterling, Virginia. 

If you'd like to chip in and help, we have also developed the 
tools that enable you to run your own single-camera CAMS sta- 
tion. This effort is coordinated by FPOA member Dave Samu- 
els. See the box on page 25 for more details. 

Each camera generates about 10 gigabytes of video per clear 
night. Software examines the footage for meteors, and it saves 
only 8-second snippets that contain moving objects. That still 
means that each camera collects about 200 megabytes of data 
per night. 

I collect the 60 cameras' video records for processing at the 
SETI Institute. One software tool maps each camera's field of 
view based on the star background. Another extracts meteor 
trajectories and speeds. Software then looks for coincidences: 
the same meteor filmed from at least two stations. I check 
the quality of each coincidence to exclude birds, bats, clouds, 
planes, and other false matches that the software has not 
already filtered out. The final product is a reliable table with all 
trajectory and orbit information, plus light curves. 




Mapping Meteoroid Orbits 



Most other all-sky meteor networks focus on catching 
fireballs that might drop meteorites on the ground. These 
record only a few meteors per night from which we can extract 
good orbits. Our goal for CAMS was to scale up the harvest. 

Most of the meteors we detect are magnitude +1 to +3, the 
same that visual observers generally see. When all the cameras 
operate as expected, we determine 100 to 300 meteoroid orbits 
on a clear night. We now have collected more than 47,000 orbits. 

CAMS is producing as many orbits each clear night as the 
original three-year tally published from a similar effort by the 
Japanese SonotaCo consortium, led by Touru Kanamori of 
Tokyo. That ongoing project started in 2007 and is expected to 
produce many more orbits in the future. The SonotaCo network 
consists of 25 amateur astronomers who operate up to 100 cam- 
eras with about a 90° field of view, recording slightly brighter 
meteors. Because of our smaller field of view and higher spatial 
resolution, CAMS measures orbits about twice as accurately. 

"Established" Showers 

Together, SonotaCo and CAMS are giving us a better picture 
than ever of which meteor showers are active around the year, 
and how showers change from year to year. 



To display the 64 showers officially established so far, I made 
15 maps for spans of several days using combined SonotaCo 
and CAMS data. Some of these maps are shown in this article; 
all can be seen at skypub.com/cams. Each colored dot is the cal- 
culated radiant of a single meteor. Color indicates the meteor's 
arrival speed, from violet for the slowest to red for the fastest. 
A group of same-colored dots indicates a shower, often with its 
three-letter designation. The maps also identify some previ- 
ously unconfirmed showers seen clearly in the CAMS data; we 
expect that these will soon receive the label "established." 

CAMS also picked up many previously unknown showers, 
and some were detected well enough to deserve "established" sta- 
tus right away. For example, six meteors with one ultra- compact 
radiant appeared within a few hours on February 4, 2011. This 
radiant was not there in earlier years. Now called the February 
Eta Draconids, this shower was caused by Earth briefly being 
hosed by the first-revolution debris trail of a previously unknown 
long-period comet. Because the bits were released only at the 
comet's last return, the radiant swarm is very concentrated. The 
stream is the first evidence of this comet's existence, and the fact 
that we were briefly showered by its first-revolution debris tail 
implies that the comet could potentially hit Earth. 



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Right Ascension 

The Ursids (URS), originating just off the bowl of the Little Dipper, 
come from the Halley-type Comet 8P/Tuttle. The December Comae 
Berenicids (COM) peak this week, but the shower is active all December 
and January. Radar observations had earlier found the daytime Omega 
Serpentids and Sigma Serpentids (OSE and SSE); the CAMS project con- 
firmed them with a few meteors seen coming from these radiants before 
dawn. There are 2,792 meteors from both SonotaCo and CAMS. 



Big 
Dipper* 



I SCORPIUS 



h 16 h 12 h 8 h 4 h 

Right Ascension 

In the year's first week, the brief but rich Quadrantid (QUA) shower 
sends medium-speed meteors radiating from near the Big Dipper. 
These originate from the asteroid-like object 2003 EH n , a dormant Jupi- 
ter-family short-period comet that suffered a breakup about 500 years 
ago. Other identified showers are the Comae Berenicids, the January 
Leonids (JLE), and the Alpha Hydrids (AHY). Note the hints of additional 
clumps. There are 2,831 meteors from SonotaCo and 381 from CAMS. 



24 September 2012 SKY & TELESCOPE 



Also, while reducing CAMS data for this article, I discovered 
a shower of very slow meteors (16.7 kilometers per second, or 
37,400 mph) radiating from Cassiopeia in early December and 
originating from a Jupiter-family comet. From what I can tell, 
nobody has ever heard of it. It has been designated the Decem- 
ber Phi Cassiopeiids (code DPC). 

Even with more than 100,000 orbits now measured by 
CAMS and SonotaCo combined, the sky is still sparsely sam- 
pled. CAMS maps of single nights are clumpy, suggesting that 
unrecognized showers lurk just below the threshold and are 
awaiting better data. In the next two years we hope that CAMS 
will bring these showers into focus. 

And we hope that readers will set up single- CAMS stations 
to create a wider surveillance network, and to help ensure that 
video surveillance of the night sky will continue. With some of 
the more interesting showers showing up only once in a while, 
the future will surely be rich with surprises. + 

Peter fenniskens is a meteor researcher with the SETI Institute and 
NASA/Ames Research Center. He chairs the IAU Task Group on 
Meteor Shower Nomenclature and is author of "the" hook on the 
topic, Meteor Showers and their Parent Comets. 




1 ¥ 

IF 



CAMS VOLUNTEERS Above: 
At the College of San Mateo, just 
south of San Francisco, California, 
observatory director Dean Drumheller 
(left) and astronomy and physics 
professor Darryl Stanford (right) 
operate the skyward-pointed CAMS 
meteor camera on the wall behind 
them (inset photo at left). 




Join Us! How to Set up a Single-CAMS Station 



Watec Wat-902H2 Ultimate 
low-light security camera 




12-mm f/1.2 fixed-focus lens 



SHOPPING LIST The main items 
you'll need to buy to join the CAMS 
project are a Watec Wat-902H2 
Ultimate low-light security camera 
(a standard weatherproof housing 
is also recommended), a 12-mm 
f/1.2 fixed-focus lens for it, and the 
EasyCap frame grabber for your 
computer's operating system. 



My colleagues and I urge amateurs to participate 
in the CAMS project. You can set up a camera at 
your own location to help determine meteoroid 
orbits with the same precision as the main part 
of CAMS, using the same tools. 

First, find at least one friend 25 to 60 miles 
away who will join the project. A single CAMS 
camera can detect about 50 meteors per clear 
night. Two cameras spaced appropriately can 
allow us to measure about 10 to 20 accurate 
orbits per night. 

Each station needs a Watec Wat-902H2 
Ultimate camera, a 12-mm f/1.2 lens for it, an 
EasyCap frame grabber, a video cable, access to 
power, and a PC. The cameras should be aimed 
at the same volume of the atmosphere about 50 
or 60 miles up. They can be mounted outside 
in a standard weatherproof security-camera 
housing (recommended), or indoors behind a 
window in a dark room. The computer needs to 
be dedicated to the project during capture hours. 
The combined cost of the camera, lens, frame 
grabber, and cable is less than $600. 

It takes about an evening's work to set up the 
system. Some experience with running Windows 
console programs and scripts is helpful. After 
that the system can work almost by itself, with 



scripts running the various software tools at the 
appropriate times. 

After everything is set up and calibrated, the 
camera will automatically begin recording each 
night. A fast multi-core computer can process 
the video as it comes in and finish a few minutes 
after recording stops at dawn. Slower computers 
will need to run the software later, which can take 
3 to 8 hours. Finishing and uploading the results 
takes less than a half hour per week of clear 
nights. The average data upload you need to 
send us is just 2 to 6 megabytes per clear night. 

See cams.seti.org/easyCAMS.html for more. 




SkyandTelescope.com September 2012 25 





If 



David Baker & Todd Ratcliff 




ie soiar sysiem s 
ild weather can make 
arth's own extremes 



em serene. 



ri 




"Can you believe this weather?" 

It's more than just small talk. Weather significantly impacts our daily 
lives. Food, clothing, shelter, heating and cooling, work, play, transporta- 
tion, communications, and health are all intimately affected by it. There 
is nothing superficial about the weather. 

Earth's dynamic weather is ultimately the product of temperature gra- 
dients. A pleasant sea breeze at the beach is created by temperature differ- 
ences between land and water. Refreshing rain showers can develop when 
unstable hot air rises up from the surface and cools, allowing the water 
in it to condense. Even on the global scale, energy imbalance between the 
equator and poles drives worldwide atmospheric circulation. 

Yet Earth's attempt to equilibrate its temperatures doesn't always go 
smoothly. Violent twisters rip through Tornado Alley when warm, moist 
air from the Gulf of Mexico collides turbulently with cool, dry air from 
the north. Lightning sizzles at 28,000°C (50,000°F, nearly five times hot- 
ter than the Sun's surface) and damages more than $1 billion in property 
each year. Tropical cyclones such as Hurricane Katrina, with sustained 
peak winds of 78 meters per second (175 mph) and an ocean storm surge 
more than 10 meters (33 feet) high, ravage unprotected coastlines and 
damage delicate ecosystems. 

Due to the chaotic nature of Earth's atmosphere, forecasting weather 
can be challenging. Small fluctuations in the atmosphere can lead to 
hefty weather changes later because of the complex ways wind, tempera- 




■■■^^l 



Left: Tracks left by dust devils' passage on Mars 
appear bluish in this color image from the Mars 
Reconnaissance Orbiter's HiRISE camera. 



ALL IMAGES OF DUST DEVIL TRAILS: NASA 



This mosaic combines 24 images of Jupiter's Great Red Spot taken by the Voyager 1 spacecraft 
on March 4, 1979, when the spacecraft was 1.9 million kilometers (1.2 million miles) from Jupiter. 







;\ 










NASA / JPL, DIGITAL PROCESSING: BIORN JONSSON 



SkyandTelescope.com September 2012 27 



Alien Storm Chasing 



Mars 
dust devil 
10 km 
(6.2 mi) - 





S&T: LEAHTISCIONE 



Martian dust devils can easily rise 10 km high, towering above similar vortices 
on Earth. Terrestrial tornadoes may extend 6 km into the sky, although that's 
something of a guess: what constitutes the top of a tornado isn't universally 
agreed upon, so height estimates vary. 







ust aevil curves as a westerly Dreez 
from the HiRISE camera on NASA's Mars Reconnaissance Orbiter 
Similar storms did more good than harm to the rovers, cleaning 
Spirit and Opportunity's solar panels of built-up dust. 



% 




Seen on March 14th this year by HiRISE, 
this gargantuan dust devil towered 
20 km (12 miles) above the Martian 
surface as it swept across the northern 
region of Amazonis Planitia. 

NASA / JPL-CALTECH / UNIV. OF ARIZONA (2) 






ture, and moisture interact. Improvements in satellite 
observations and computer models now provide three- day 
forecasts with higher accuracy than the one- day forecasts 
of 1980, but some of the most severe weather can still only 
be predicted a few minutes in advance. 

Weather on Earth certainly can be intense and capri- 
cious. But how does it compare with weather on other 
planets in our solar system? Is our weather truly extreme? 
The answers to these questions can make Earth's atmo- 
spheric tantrums look downright placid, and they might 
even have implications for the evolution of life throughout 
the solar system. 

Vacuum Cleaners of Dusty Mars 

Mars has long been known for its nasty weather. When 
Mariner 9 arrived at Mars in 1971, a global dust storm 
raged across the planet and obscured almost the entire 
surface. Only a handful of peaks rose above the clouds, 
including Olympus Mons, the highest mountain in the 
solar system at an elevation of 27 km (17 miles) above 
Mars's average radius. Fortunately for our space probes, 
these planet-wide storms are relatively rare. Only eight 
have been recorded since 1956, the most recent in 2007. 

It's really the smaller dust storms that rule the Red 
Planet. Dust devils (or traces of them) appear almost 
everywhere on Mars, from the deep Hellas Basin to the 
lofty Tharsis volcanic region. The Mars Orbiter Camera 
(MOC) aboard NASA's Mars Global Surveyor spacecraft 
detected nearly 11,500 active dust devils over four Martian 
years. The Spirit rover observed 533 dust devils in Gusev 
Crater during one season alone. 

Shorter Martian vortices can be measured from lander 
images taken at the surface, but the tallest dust devils 
must be measured from above by orbiting spacecraft using 
shadow lengths and the angle of solar incidence. These 
observations show that "small" is a relative term: some 
dust devils can be more than a mile wide and reach higher 
than Mount Everest. Winds from these blowing behemoths 
travel the length of a football field in about a second. 

Like tornadoes on Earth, these rotating giants leave 
their destructive mark on the Martian surface. As they 
sweep across the landscape, Mars's dust devils vacuum 
up a layer of red dust and expose the darker underlying 
substrate. Faster dust devils leave linear streaks, while 
slower vortices trace looping, curved paths. Dark tracks 
often remain visible for a month or longer, etching Mars 
with crisscrossed streaks. 

But similarities between tornadoes and dust devils end 
with their signatures on the landscape. The fundamental 



Watch roving Martian dust devils and hear light- 
ning on Saturn at skypub.com/planetstorm. 



The dust-devil-swept dunes near Mars's south pole look more like 
spun silk than a barren, alien landscape in this HiRISE image. 



Shown in natural color in these Hubble Space Telescope 
images, a global dust storm engulfed Mars in 2001 as the 
southern hemisphere's spring began. Only a few clouds 
appear in the left image, taken in June; by September, the 
storm had covered the planet for nearly two months. 



dynamics of these whirling cousins are quite different. 
Tornadoes are strongly influenced by cold downdrafts 
that are enhanced by evaporative cooling behind the 
storm's core. These rear flank downdrafts wrap around 
the cyclone and squeeze rotating air into tighter rolls. In 
contrast, dust devils develop from a combination of warm 
updrafts and the uneven convergence of swirling horizon- 
tal winds (those pesky temperature gradients again). 

Although tornadoes on Earth are relatively infrequent, 
Martian dust devils are remarkably commonplace. Based 
on the abundance of dark tracks on the Red Planet, many 
dust devils probably race across the barren surface every 
day. Yet because of the low atmospheric surface pressure 
on Mars, each dust devil individually packs a light punch 
— high winds but relatively small momentum. The winds 
are strong enough to lift dust particles (aided by the lower 
gravity of Mars, page 16), but an astronaut would barely 
feel them. Even a slight gust on Earth exerts more force. 

The collective impact of these red whirling dervishes 
extends well beyond their local domain. Dust devils may 
contribute significantly to global atmospheric haze and 
the Martian dust cycle. Like clawing fingernails scraping 
away the skin of Mars, these incessant planetary vacuum 
cleaners are devilishly extreme. 

Shocking Superbolts of Saturn 

Bolts of lightning can be found sizzling throughout the 
solar system. All major planets with atmospheres (sorry, 
Mercury) show evidence of such electrical discharges. But 
nowhere does lightning crackle more powerfully than on 
the ringed giant Saturn. 

Remarkably for such a common phenomenon, scien- 
tists don't fully understand how lightning forms. The 
leading hypothesis involves collisions of ice particles 
within a thunderstorm. As hail and softer snow pellets 
fall through the clouds, these large ice projectiles scav- 
enge electrons from smaller ice crystals. The lower clouds 
become negatively charged, while the upper clouds swell 
with positively charged ice crystals. When roughly 100 



million volts build up in Earth's atmosphere, electrons 
discharge across the cloud (and sometimes to the ground) 
to paint dramatic displays of light and sound. 

Because of water's electric dipole (the molecule is 
negatively charged on one side and positively charged on 
the other), water ice is particularly effective at stealing 
electrons. For this reason, planetary scientists usually 
focus on water clouds as the source of lightning. But 
these brilliant flashes of visible light are often hard to 
detect, especially on a planet's dayside or coming from 
water clouds buried deep within its atmosphere. 

Luckily, lightning emits electromagnetic radiation 
at a variety of wavelengths. Radio waves in particular 
can travel great distances with minor attenuation. The 
ionosphere traps the low-frequency "whistler modes," 
named for their whistling sounds when heard over a radio 
receiver. Higher-frequency radio waves escape to space 
and sound like annoying static on an AM radio when 
intercepted by spacecraft instrumentation. 

As NASA's Cassini spacecraft flew by Earth in 1999 for 
a gravitational assist on its way to Saturn, its Radio and 
Plasma Wave Science (RPWS) instrument detected radio 
bursts from lightning at a distance of 89,000 km above our 
home planet. The same instrument measured similar radio 
pulses at Saturn (called Saturn Electrostatic Discharges, 
or SEDs) much farther away, roughly 160 million km from 
the ringed planet. Since intensity drops off as the distance 
squared, Saturn's superbolts must be about a million times 
more energetic. Imagine for a moment what it must be like 
inside one of these superstorms — violent updrafts stron- 
ger than any on Earth, huge hailstones creating whopping 
charge separation, and epic lightning bolts perhaps as wide 
as the Washington Monument. 

Until recently, most SEDs measured by Cassini origi- 
nated from the southern hemisphere in a band at 35° 




The Dragon Storm, with bizarre arms in Saturn's "Storm Alley," 
released shockingly strong radio emissions in September 2004, a 
product of intense electrical activity within the storm. 



Alien Storm Chasing 



Super-rotating Tropical Titan 

It may seem strange to call Saturn's largest moon "tropical." Titan 
receives only about 1% of the sunlight that reaches Earth, and its high- 
est surface temperature is a bone-chilling -178°C (-288°F). But Titan's 
weather patterns closely resemble Earth's tropics and, perhaps even 
more so, those of blazing-hot Venus. 

Titan is tidally locked to Saturn in a synchronous rotation of 15.9 
days, meaning the same side of Titan always faces the ringed planet, 
just as the Moon always shows the same face to Earth. This relatively 
slow rotation makes Titan's atmosphere behave differently than the 
atmospheres of planetary bodies with short solar days. The dominant 
north-south circulation pattern on Titan consists of Hadley cells that 
extend from the latitude of maximum heating (e.g., the equator at 
equinox) all the way to the poles. For rapidly rotating planets, however, 
strong Coriolis forces prevent Hadley cells from reaching the poles. On 
Earth, Hadley cells only extend to ±30° latitude — a.k.a. the tropics. 

Dynamic weather events at mid-latitudes on Earth, such as cyclonic 
low-pressure systems, undulating jet streams, and warm and cold 
fronts, are completely absent on Titan. Instead, the entire moon expe- 
riences weather characteristic of the terrestrial tropics (minus the hur- 
ricanes), including tranquil breezes and occasional methane showers. 

But things are not calm everywhere on Titan. In the stratosphere, 
winds scream around the moon at over 100 meters per second (220 
mph), rushing in the same direction as Titan rotates. Yet Titan's solid 
body only rotates at about 12 mps (26 mph). This prograde gusting 
means that Titan's atmosphere "superrotates," twirling completely 
around the moon in 1.9 days, roughly one-tenth the moon's synchro- 
nous rotation period. This extreme behavior is found on another slowly 
rotating body: Venus's atmosphere superrotates every 4 days com- 
pared to its solid-body rotation of 243 days. 

The exact cause of superrotation on Titan and Venus remains unre- 
solved. One leading hypothesis, supported by computer simulations, 
suggests that planet-scale atmospheric waves might transport signifi- 
cant amounts of momentum to the upper atmosphere, accelerating 
the winds to super-speeds. 




south latitude called Storm Alley. As the season transi- 
tions to northern spring, even stronger superstorms are 
brewing in the northern hemisphere. A massive "white" 
storm develops roughly every 30 years when Saturn's 
north pole tips toward the Sun. These "Great White 
Spots" (May issue, page 20) have been observed only six 
times since first being detected in 1876. But, as the sixth 
storm's 10-year-early arrival proved, Saturn's weather 
isn't so easy to predict. SEDs from this superstorm 
(which evolved into a giant, turbulent snake completely 
encircling Saturn) were so frequent that Cassini's RPWS 
instrument sometimes could not even resolve individual 
lightning strokes in the cacophony. The most shocking 
developments are surely yet to come. 

Jupiter's Permanent Storm 

Devastating hurricanes may roil for weeks on Earth. Elec- 
trifying storms on Saturn may blaze for months. But for 
longevity, Jupiter's Great Red Spot takes the prize. 

Italian-French scientist Jean Dominique (Giovanni 
Domenico) Cassini, whose namesake spacecraft is now 
orbiting Saturn, first definitively observed this "perma- 
nent" spot in Jupiter's southern hemisphere in 1665. 
(Robert Hooke is often credited with discovering the 
Great Red Spot in 1664, but Hooke's "small spot" possibly 
occurred in the northern hemisphere and could have 
been a shadow transit of a satellite.) Observing records 
are sparse for the 165 years following Cassini's death in 
1712, but the giant storm has been systematically recorded 
since 1878. This makes the Great Red Spot at least 134 and 
possibly 347 years of age, and perhaps even older. 

The exact reason for the 165 -year gap in documentation 
may never be known. It's possible that the spot Cassini 
saw dissipated and a different storm (the one we see 
today) developed more than a century later. Or maybe the 
blemish simply faded in color — Hubble Space Telescope 
images show variations from deep red to light salmon in 
the span of just a few years — and it could not have been 
detected with the telescopes of the 18th and 19th centuries. 

Regardless of its exact age, the Great Red Spot is 
imposing. The anticyclonic high-pressure system towers 
8 km over the surrounding cloud tops (nearly as high as 
Mount Everest). Winds whip around the giant whirlpool 
at 190 mps (425 mph), faster than any winds ever recorded 
on our home planet. Almost three Earths could fit inside 
the unrelenting maelstrom. 

Why has this giant vortex lived for so long? First, intense 
internal heating from gravitational contraction fuels Jupi- 
ter's turbulent weather. The cloud tops receive 70% more 
energy from the planet's interior than from the Sun. Pow- 
erful thunderstorms (many of which have been detected 





near the Great Red Spot) transport much of this heat from 
the deep interior to help equilibrate the energy imbalance. 

Second, friction caused by land masses on Earth 
destroys hurricanes when they hit shore, limiting their 
lifetimes to a few weeks at most. Without a solid surface, 
Jupiter's interior exerts much less drag on the atmosphere. 
The end result is that Jovian storms can churn for years. 

But the Great Red Spot isn't your typical low- friction 
Jovian storm. This vicious anticyclone is trapped between 
two high-speed jet streams, a westward jet to the north 
and an eastward jet to the south. Computer simulations 
show that multiple vortices, not just one, develop at the 
interface of such strong opposing jets. These vortices 
eventually consolidate into one big stable vortex. In other 
words, the Great Red Spot eats smaller spots to maintain 
its impressive physique. 

Similar spots have been detected on the other giant 
planets — white spots on Saturn, white spots on Uranus 
(see page 54), and the Great Dark Spot on Neptune — but 
none of them seems to have the stamina of the Great Red 
Spot. Why such a long-lived vortex only on Jupiter? It's an 
intriguing puzzle, especially considering that Saturn also 
has strong internal heating and wicked jet streams. 

Launched in August 2011 and set to arrive at Jupiter in 
2016, NASA's Juno spacecraft will investigate the compo- 
sition, structure, and dynamics of the Jovian atmosphere 
(S&T: September 2011 issue, page 18). A six-band micro- 
wave radiometer will penetrate the cloud tops to examine 
the deep atmosphere down to the 200-bar pressure level 
(about 400 km below the ammonia clouds). An infrared 
spectrometer will map deep water clouds and thunder- 
storm development. This important new mission may 
finally reveal the secrets of Jupiter's eternal storm. 

Nature's Wrath or Nature's Nurture? 

Mars, Saturn, and Jupiter all exhibit weather beyond the 
norm, with armadas of colossal dust devils, superbolts of 
shocking proportions, and a raging red tempest that may 
never die. For successful space exploration, in situ space- 
craft must be built to investigate and withstand these 
types of extreme weather. 

Weather is a critical consideration when design- 



The Great Red Spot devours a smaller vortex in this sequence 
from 2008. Three red spots can be seen in these Hubble Space 
Telescope images: Great Red Spot, Little Red Spot (same latitude 
but to the west of the great one), and Red Spot Junior (south of the 
other two spots). In the last panel, the Little Red Spot (now on the 
east side) gets swallowed by its giant neighbor. 



ing space missions. Scientists pored over images for 
evidence of dangerous dust devils when choosing the 
landing site near the north pole for NASA's 2008 Phoenix 
Mars Lander. Lightning on Saturn poses huge risks for 
electronics on future atmospheric probes, and the never- 
ending roller coaster ride of the Great Red Spot probably 
would shred a typical weather balloon. Human explora- 
tion in these violent conditions is almost unthinkable. 

Brutal weather on other planets may make it difficult 
for any life form (as we know it) to survive. Consider a 
hypothetical microbe trying to get a flagellum-hold on 
Mars. Just when it gets attached to a nice piece of dust 
with some water ice nearby, whoosh! a dust devil merci- 
lessly relocates it to a fatal locale. Nature's wrath may not 
be a microbe's friend. 

On the other hand, extreme weather might encourage 
life. Hurricanes rejuvenate coral reefs, bring precipitation 
to drought- stricken areas, and flush out toxic pollutants 
from estuaries. Lightning in Earth's early atmosphere 
might even have sparked life by creating organic com- 
pounds such as amino acids. Maybe abnormal weather on 
other planets could prime their environments for life. We 
won't know until we look. 

In terms of absolute wind speeds, vortex size, or elec- 
trical power, Earth may not have the wildest weather in 
our solar system. But for supporting life, there is nothing 
third-rate about the Third Rock. Earth's weather might 
just be the perfect storm. How wild is that? 

David Baker is chair of the Physics Department at Austin 
College in Sherman, Texas; Todd Ratcliffis a planetary 
scientist at NASA's Jet Propulsion Laboratory in Pasadena, 
California. Their recent hook, The 50 Most Extreme Places 
in Our Solar System, explores some truly wild places. You 
can vote for your favorite at ExtremeSolarSystem.com. 



rs, even fields of sar 
rthemarkofpai 



~uilt up inside 
devils. 





SkyandTeles< 


op..™ S , pl 


temher 2012 31 









% Mini-World Rendezvous 



Jim Bell 



Dawn is 
revealing the 
history of Vesta, 
a unique world 
that is part 
asteroid and 
part planet. 



What is the recipe for building a planet? What kinds of 
building blocks are needed, and what forces hold them 
together or try to tear them apart? How does a planet's 
structure and composition change as it grows? What clues 
still preserved on planets or planetary fragments can help 
us understand their origins? These are the questions that 
NASA's Dawn mission was designed to answer. 

Dawn, the ninth mission in NASA's "faster, better, 
cheaper" Discovery series, is a spacecraft built to study 
two of the solar system's largest asteroids: Vesta and 
Ceres (S&T: November 2011, page 32). These two bodies 
alone represent a little over 40% of the mass of the main 
asteroid belt. Previous telescopic observations have 
hinted that rather than being primitive, unaltered objects 
like most smaller asteroids, Vesta and Ceres may be 
transitional protoplanets: part asteroid, part planet. If so, 
studying them up close could provide important clues 
about how planets such as Earth came to be. 

Telescopic observations revealed Vesta to be 
dramatically different in color and surface mineral 
properties than any other large asteroid. Specifically, 
Vesta shows evidence of some of the kinds of basaltic 
minerals that we find in volcanic lava flows on Earth and 
other terrestrial planets. Scientists noticed that Vesta's 
infrared spectrum is remarkably similar to the spectra 
of a class of meteorites called the HEDs (Howardites, 



Eucrites, and Diogenites), ancient samples of which 
apparently started out as volcanic rocks within the crust 
or mantle of a large terrestrial planet or protoplanet and 
that were later transported to Earth by impacts. More 
recently, astronomers have found many smaller asteroids 
with Vesta-like spectral properties (Vestoids) traveling 
as families of small bodies in orbits related to Vesta's. 
Hubble Space Telescope imaging revealed an enormous 
hole at Vesta's south pole — as if a large chunk of the 
asteroid had been blasted off in a giant impact. 

Dawn was launched in September 2007, cruised past 
Mars in 2009 for a gravitational assist, and settled into 
orbit around Vesta in July 2011. The spacecraft's ion- 
propulsion system, and a cautious, systematic mission- 
operations approach, have enabled the Dawn science 
team to study the asteroid's surface from successively 
lower orbits. A high-resolution color camera has obtained 
thousands of images down to resolutions of less than 
100 meters, and a visible to near-infrared spectrometer 
has acquired millions of spectra of the surface down to 
resolutions smaller than 1 kilometer. These images and 
spectra have enabled the Dawn science team to make 
some exciting discoveries about Vesta's geology and 
mineralogy. Tracking of the spacecraft's orbit has enabled 
accurate measurements of Vesta's mass as well as detailed 
estimates of its interior structure. 




ALL IMAGES COURTESY NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA UNLESS OTHERWISE CREDITED 



Perhaps the most important finding is Dawn's 
discovery of not one but two enormous impact basins 
near Vesta's south pole. The largest, called Rheasilvia, is 
about 500 km (310 miles) across and nearly 20 km deep — 
more than enough volume to account for the HEDs and 
Vestoids. Rheasilvia has an enormous interior mound of 
rock nearly 200 km across and more than 20 km high, 
making it one of the solar system's highest mountains. 
We think it's the rebounded central peak created by the 
impact. Rheasilvia occurs on top of (and significantly 
overlapping) a slightly smaller (400-km-wide) basin 
called Veneneia. Using computer models of impact crater 
counts, scientists have estimated Rheasilvia's age as about 
1 billion years and Veneneia's as about 2 billion years. The 
fact that they overlap near the south pole appears to be a 
cosmic coincidence. These two giant holes cause Vesta to 
appear distinctly flattened relative to a perfect sphere, as 
first noticed in HST images. 

Indeed, impact craters dominate Vesta's shape and 
geology. Scientists have discovered five other basins larger 
than 150 km across, and thousands of smaller craters 
pockmark the surface. Vesta's surface is saturated with 
craters, meaning that the surface is so thoroughly covered 
by them that any new ones that form will wipe out 

Left: This close-up of Vesta's south polar peak, with the asteroid's 
curvature removed, is based on a shape model. Right: A 
colorized terrain map of Vesta's surface illustrates the variety of 
topographical features. High areas are red and white, low ones 
are violet and blue. The equatorial grooves are easily visible. 



Above left: The south polar peak rises a whopping 15 km above 
the crater floor. Above: This image of Vesta's southern hemi- 
sphere highlights 10-km-wide equatorial grooves. The troughs are 
concentric to Vesta's two south polar impact basin and probably 
resulted from shock waves produced by the two giant collisions. 

enough old ones to keep the total number about the same. 
The two large south polar impacts appear to have reset 
the geology of the southern hemisphere, which has fewer 
craters overall than the part of the northern hemisphere 
imaged so far. All of these impacts have created a shallow, 
well-mixed layer of fine-grained impact debris — a regolith 
— overlying the asteroid's dense bedrock subsurface. 
Vesta's regolith is similar in some ways to the powdery, 
impact- created upper surfaces found on other asteroids 
such as 433 Eros, as well as on the Moon. 

Dawn has also revealed lots of evidence for erosion, 
mostly in the form of landslides and slumps of material 




SkyandTelescope.com September 2012 33 



Mini-World Rendezvous 




flowing down crater rims and walls due to Vesta's weak 
gravity (less than Vsoth of Earth's gravity). Dawn has also 
detected a series of long, deep grooves and troughs that 
circle much of Vesta's equator. Although scientists are 
still debating the details of how these tectonic features 
formed, they are clearly concentric to the two large south 
polar basins, and so they must be related. Perhaps the 
shock waves generated by these enormous impacts helped 
to form these giant cracks. 

In addition to the geologic diversity, Vesta also 
exhibits the widest range of albedo (brightness), color, 
and infrared spectroscopic diversity of any asteroid yet 
encountered by a spacecraft. These variations indicate 
that Vesta is not a primitive world but instead that 
its surface materials have been processed by internal 
or external forces that can convert a planet's original 
inventory of rocks and minerals into other forms. 

Indeed, a planet's mineral inventory provides 
information about the specific processes and 
environments that shaped that world. For Vesta, the 
composition consists of mixtures of basaltic volcanic 
minerals such as pyroxene — a common volcanic mineral 
on the terrestrial planets — as well as darker minerals 
whose types are more difficult to identify. The volcanic 
minerals match the compositions of the HED meteorites, 
which confirms the link between Vesta, the Vestoids, 
and this class of meteorite samples. The darker mineral 
phases could either be more iron-rich volcanic minerals, 
or perhaps more carbon-bearing materials from primitive 
(unprocessed) carbon-rich impactors that slammed into 



30 km 



20 km 



10 km 



km 



Mountains Compared 




100 k 



200 kn 



300 km 



16° N 



24° S 



64° S 



! 





180 360 

Top: The central peak in Rheasilvia rivals Mars's Olympus Mons as one of 
the solar system's highest mountains. Above: Green areas in this false-color 
map of Vesta indicate high volcanic-mineral content. Redder and bluer areas 
highlight regions with the strongest color differences. 



The blue area around the 24-km-wide crater at me u| 
left of this false-color image indicates material ejected 
impact. This terrain is near the edge of Rheasilvia. 




34 September 2012 SKY & TELESCOPE 




This image from Vesta's largely unexplored northern hemisphere 
shows its surface etched with subtle grooves, some of which run 
for dozens of kilometers but are less than 1 km wide. Shifting 
regolith on Vesta's surface may be responsible for the grooves. 

Vesta over time. If it's the latter, these may be the kinds of 
organic materials that helped seed the ingredients for life 
on Earth when delivered here by impacts. 

Somewhat surprisingly given Vesta's mineral 
composition, Dawn has seen little or no evidence for lava 
flows, dikes, cones, domes, or other volcanic structures. 
Maybe these features once existed on Vesta, but they have 
since been destroyed by the onslaught of impact cratering. 
Or perhaps Vesta's volcanism has always been hidden 
below the surface. 

Looking at the bigger picture, the distribution of 
surface and impact- excavated minerals indicates that 
Vesta is a differentiated world. That is, the asteroid must 
have been at least partially melted early in its history, 
perhaps even with a melted ocean of magma floating at 
the surface. This enabled heavier iron- and nickel-bearing 
materials to sink and lighter silica- and oxygen-bearing 
materials to rise, segregating the interior into a core, 
mantle, and crust. Vesta's initial melting probably came 
from the combined heat of radioactive elements within 
the interior and the heat energy of innumerable impacts. 

Differentiation into a core, mantle, and crust, and 
evidence for interior volcanic processes, are typical 
hallmarks of a terrestrial planet. Indeed, Dawn's 
measurements of Vesta's mass, combined with an 

From an altitude of 272 km, Dawn examined a portion of this 
40-km-wide crater that's located a few degrees south of the 
equator. The crater's sharp rim indicates a fairly recent impact, 
and its interior walls feature a variety of bright outcroppings. 



estimate of the asteroid's volume from stereo imaging 
data, indicate that it has a high rocky and metallic density 
of about 3.4 grams/cm 3 , comparable to the densities of the 
Moon and Mars. By some definitions, then, Vesta should 
be considered a planet, albeit a small one. 

Some scientists think that Vesta may in fact be a 
protoplanet. In other words, Vesta may be the preserved 
remnant of a transitional class of planetary objects — it's 
no longer a primitive asteroid but it's not quite a classical 
terrestrial planet either. The world's meteorite collections 
reveal that we have samples from perhaps 100 or more 
such transitional protoplanets, including the parent body 
of the HEDs. Vesta is almost certainly that parent body, 
but it's the only object we know of in the solar system for 
which we can make that kind of claim. Vesta may thus 
be the last of its kind, a rare survivor of the violent era of 
planet formation. 

Dawn's mission at Vesta will continue through most of 
the summer of 2012. The science team will collect images 
and other data of Vesta's far northern hemisphere, which 
has been in winter darkness for most of the mission so 
far. Scientists are only now analyzing the compositional 
results from the gamma-ray and neutron spectrometer, 
since it took such a long time to build up the required 
signal levels during the mission's recent low-orbital- 
altitude phase. Will there be additional geological or 
mineralogical surprises in Vesta's more ancient northern 
hemisphere? Will the gamma-ray or neutron data reveal 
the presence of hydrogen-bearing materials (hydrated 
minerals or even ices) or other volatiles in the subsurface? 




SkyandTelescope.com September 2012 35 



Mini-World Rendezvous 



According to the current plan, in late August 2012, the 
spacecraft will be gently guided out of Vesta's orbit and on 
to its next encounter: a February 2015 orbital rendezvous 
with Ceres, the solar system's largest asteroid. Based on 
telescopic observations, Ceres is very different from Vesta, 
with a lower density and likely a more icy composition. 
What surprises await us there? Will Ceres prove to be a 
surviving protoplanet like Vesta, or something entirely 
different? I can't wait to find out! + 

Contributing editor Jim Bell is a professor of astronomy and 
planetary science at Arizona State University. He was involved 
in the NEAR- Shoemaker mission to asteroid 433 Eros and 
is the lead scientist for the Mars Exploration Rover Pancam 
instruments. When not studying planetary surfaces, he roots for 
the Boston Red Sox and paddles Hawaiian outrigger canoes. 




By counting craters, scientists can estimate the ages of the two 
giant impact basins around Vesta's south pole. As this false-color 
map shows, the two basins overlap — a cosmic coincidence. 




36 September 2012 SKY & TELESCOPE 




SkyandTelescope.com September 20 1 2 37 



S&T Test Report Alan Dyer 



The 60 Da: 

Canon's Astrophoto DSLR 




Canon 60Da DSLR 

U.S. price: $1,499.95 

www.usa.canon.com; available from dealers worldwide 



Canon surprised the astronomy world with a version of its 6oD camera 
designed specifically for astrophotography. How well does it work? 



NO ONE EXPECTED THIS CAMERA. Even the 
Canon rumor sites on the internet were silent on its 
pending introduction. But last April Canon unveiled the 
60Da, a version of its popular mid-range 60D designed 
specifically for astrophotography. 

The 60Da follows in the tradition of the landmark 
Canon 20Da, an astronomical version of the 20D that 
could take excellent long- exposure images of the night 
sky. The Canon 20Da debuted in 2005 (I reviewed it in this 
magazine's November 2005 issue, page 84), but the cam- 
era was soon discontinued and not replaced until now. 

The 60Da is aimed specifically at the astronomy mar- 
ket. Several years of DSLR improvement put it well ahead 
of the old 20Da. The 60Da has an 18-megapixel sensor 
in place of the 8-megapixel sensor of the 20Da, yielding 
much higher resolution. The 60Da has a modern, large, 



articulated LCD screen. And the 60Da offers a variety of 
movie modes, including one ideal for shooting planets. 
DSLRs have come a long way since 2005. 

The 60Da Advantage 

The original 20Da had the first implementation of a feature 
now common on most DSLRs: live focus or "live view." 
This allows focusing with a magnified view of a live image 
on the LCD screen, which is a far more accurate method 
than trying to focus using a dim optical viewfinder. 

The feature that makes the 60Da unique is a special 
filter in front of the camera's sensor. All digital sensors 

The Canon 60Da comes with the usual Canon accessories and cables, 
but it also includes the highly useful N3-to-E3 remote cable-release 
adapter, and an AC power supply. All photographs by the author. 



38 September 2012 SKY &, TELESCOPE 



are sensitive to infrared (IR) light that lenses typically 
can't bring to focus properly and they are thus fitted with 
IR-blocking filters. Unfortunately, the standard filters also 
block the red hydrogen- alpha (H-alpha) light of emission 
nebulae, which is important for astrophotography. 

The 60Da still has an IR-blocking filter, but it is a spe- 
cial one that transmits a higher amount of visible H-alpha 
light. This is essential for anyone wanting to capture rich, 
colorful nebulae with a DSLR. Canon states that the 60Da's 
filter transmits three times the level of H-alpha light as the 
standard 60D and 1.5 times what the old 20Da transmitted. 

I no longer own a 20Da to make comparisons, but 
I tested the 60Da against a Canon 7D (with the same 
18-megapixel sensor as the 60Da) and a Canon 5D Mkll 
that was modified for astronomical photography by 
Hutech Corporation (its IR-blocking filter was replaced 
with a Hutech Type lb filter). 

My overall conclusion is that the 60Da behaved simi- 
larly to the original 20Da — it recorded red nebulae with 
more depth and detail than did the 7D, but not as well as 
the Hutech-modified 5D Mkll. On the other hand, the 
60Da is factory warranted by Canon and offers nearly nor- 
mal color balance for conventional "daytime" photography 
(something the modified camera doesn't do). 

Color Balance 

The number one question prospective buyers have had 
about the 60Da is whether it can take normal daytime 
photos. Many backyard astronomers looking for a DSLR 
want it to serve all purposes (making it easier to justify the 




The camera's large, swing-out LCD screen provides neck- 
and back-saving convenience when framing, focusing, and 
reviewing images at the telescope. 

expense). They are reluctant to spend $1,500 or more on a 
camera dedicated solely for astro -imaging or whose images 
are compromised for anything other than glowing nebulae. 

In the 60Da's instruction sheet, Canon warns in bold 
type that "...shooting normal subjects with this camera is 
not recommended." I found this statement to be far too 
conservative — the camera worked just fine for normal 
daytime shooting. 

Changing the IR-blocking filter of a camera inevitably 
means compromising its color balance. My modified 5D 
Mkll delivers daytime images that are very pink. Although 
this can be corrected in later processing (when shooting 
RAW images), the color balance never looks quite right. 

With the 60Da's special IR-blocking filter, however, I 



Shot in quick succession on a night with high clouds drifting through, these images (processed as identically as possible) are 4-minute exposures 
at f/2.8 and ISO 800. The 7D and 60Da images were taken with a 135-mm telephoto lens, while the larger sensor of the 5D Mkll needed a 200-mm 
lens to capture a similar field of view. 




Canon 7D — The off-the-shelf 7D does a 
respectable job of recording nebulosity, but 
red nebulae appear pale and more magenta 
than with the other cameras. 



Canon 60Da — Red emission nebulosity 
records with a deeper red tint, and faint 
outlying bits of nebulosity missed by the 7D 
are picked up by the 60Da. 




Hutech-modified Canon 5D Mkll — This 
camera records the most nebulosity by far, 
though its images require much more color 
correction during processing. 



SkyandTelescope.com September 2012 39 



S&T Test Report 



These similarly processed images of the M8 region of Sagittarius are 90-second exposures at ISO 1600 taken in quick succession with the same lenses 
used for the accompanying North America Nebula set. They show sensitivity to red emission and blue reflection nebulae. 




Canon 7D — The off-the-shelf 7D again does 
a good job, but does present red nebulae with 
a paler tone, and it barely records some of the 
faintest wisps of nebulosity. 



Canon 60Da — The 60Da comes closer to 
matching the modified 5D Mkll for tonal rich- 
ness in this colorful area of sky. Emission nebu- 
lae are recorded as redder and more saturated 
than with the stock 7D. 



Hutech-modified Canon 5D Mkll — Subtle outly- 
ing bits of nebulosity east of the Lagoon Nebula 
and north of the blue Trifid Nebula record with 
more intensity than in the other cameras. 



found that even without any post process- 
ing, daylight shots looked quite natural. 
Sunlit scenes might take on a slightly 
warmer tone, but nothing that's objection- 
able or even noticeable without a direct 
comparison. Sunset and twilight shots 
with the 60Da definitely looked redder, but 
to their advantage. The 60Da is thus more 
than a camera for just deep-sky imaging. 

Noise Levels 

As a rule with digital cameras, the smaller 
the pixels the higher the image noise, 
which becomes especially apparent in 
low-light pictures. Despite its relatively 
small 4. 3 -micron pixels, the 60Da, with its 
internal DIGIC IV processing, delivered 
images with low noise. Shots taken with 
my trio of test cameras showed that the 
60Da had slightly lower noise than the 
Canon 7D, but not as low as the hallmark 
Canon 5D Mkll, with its larger, 6.4-micron 
pixels. The difference between the 60Da 
and 5D Mkll in RAW images appeared to 
be about one f/stop. For example, noise in 
60Da shots taken at ISO 1600 looked simi- 
lar to the noise in 5D Mkll shots taken at 
ISO 3200. 

In previous testing (before I sold my 



much-loved Canon 20Da cameras), I 
found that the 5D Mkll and 20Da had 
similar levels of noise and definitely set 
the standard for DLSRs. The 60Da comes 
within an f-stop of that standard, a fair 
trade-off for the 60Da's increased resolu- 
tion from its smaller pixels. Any residual 
noise is easily smoothed out with image- 
processing software. Although the 60Da 
can be set to as high as ISO 12,800, 1 
found images at that setting too noisy to 
be useful for anything more than finding 
and framing deep-sky targets. 

In room-temperature tests, 8-minute, 
high-ISO exposures taken with the lens 
capped (to record nothing but black) 
showed uniform neutral backgrounds 
with no sign of the edge glows, color 
shifts, or serious banding exhibited by 
some other DSLR cameras I've tested. This 
is performance absolutely critical to any 
DSLR camera intended for astro-imaging. 

Movie Modes 

The 60Da offers a choice of movie modes, 
including 1,920 x 1,080 HD at 30 frames 
per second. These are useful for fast- 
changing sky phenomena and for tours 
around the Moon. A mode that the 60Da 



shares with the 60D (but lacking in Can- 
on's higher-end cameras) is Movie Crop, 
which records a movie from the sensor's 
central 640 x 480 pixels at 1:1 resolution 
(the pixels are not binned or downsampled 
as they are in other movie modes). 

Although the field of view and frame 
size are small in Movie Crop mode, they 
are sufficient to record close-ups of the 
Moon and planets, thus turning the 60Da 
into a 60 frame-per-second planetary cam- 



The 60Da's Movie Crop mode turns the 
DSLR into a high-resolution planetary 
camera that records 60 frames per second 
at 1:1 pixel resolution. This daytime Venus 
image from last June 9th is a single frame 
grabbed from a movie taken with a 130- 
mm f/6 refractor and 2x Barlow. 



40 September 2012 SKY & TELESCOPE 



era. Coupling the 60Da's 4.3 -micron pixels 
to a telescope with a focal length of 2,000 
mm yields an image scale of 0.4 arcsecond 
per pixel, ideal for planetary imaging. 

The convenience of Movie Crop mode 
is that the movie is recorded directly to the 
camera's SD memory card, without the 
need for an external computer. (Jerry Lodri- 
guss wrote more about this type of plan- 
etary imaging in last May's issue, page 72.) 
This mode extends the utility of the 60Da 
(and 60D) to shooting planets, a subject up 
to now far from ideal for DSLR cameras. 

Other Features 

Having used Canon's hefty 5D Mkll and 
7D cameras for several years, I was pleas- 
antly surprised at the compactness and 
light weight of the 60Da. At 675 grams (24 
ounces), this is a camera that won't bur- 
den small telescopes on light mounts. 

A wonderful feature introduced with the 
60D is an articulated LCD screen. Having 
spent many nights crouching and peering 
up at a DSLR's screen to focus and frame a 
target, I found the swing- out screen of the 
60Da to be a godsend. If only because of 
its convenience, I found myself using the 
60Da for all kinds of sky shooting. It soon 
became my favorite camera. 

Unique to the 60Da is the inclusion of an 
adapter cable that allows Canon's optional 
TC-80N3 interval timer to connect to the 
camera. Like the lower- cost Canon Rebel 
cameras, the 60Da uses a small E3 jack for 
its remote controls. But Canon's upper- end 
cameras have a proprietary N3 jack. The 
60Da's adapter cable allows the use of these 



N3 accessories. This is a real plus for those 
of us loyal to Canon's TC-80N3 timer, which 
enables the user to set up a series of shots 
that are triggered automatically. 

The 60Da uses Canon's LP-E6 battery, 
the same as in the higher-end Canon 7D 
and 5D Mkll/Mklll cameras. In addition, 
the 60Da comes with Canon's ACK-E6 
power supply, which allows the camera to 
operate off AC power. This is noteworthy 
since the ACK-E6 is a $150 optional acces- 
sory with other Canon cameras. 

Any Caveats? 

If there is any concern with the 60Da, it is 
how long it might remain on the market. 
The 20Da lasted barely a year before it was 
discontinued. The non-astronomical 60D 
was released in mid-2010, ages ago in DSLR 
years. Will it be replaced soon with a 70D 
sporting more features and will there be a 
corresponding astronomical version? 

On the other hand, as astrophotogra- 
phers, we don't need many of the latest 
features offered by new cameras, such 
as multi-point autofocusing, rapid-fire 
continuous shooting, or touchscreen 
controls. And for the most part we do not 
need more megapixels, certainly not at the 
cost of noise — the paramount concern of 
astrophotographers . 

What would be nice is lower noise at 
higher ISO settings, allowing us to grab 
clean images in ever-shorter exposure 
times, perhaps without guiding or even 
tracking the sky. The Canon 60Da is 
certainly capable of excellent high-ISO 
performance, and it offers all the features 



most astrophotographers will need for 
many years of shooting. Furthermore, 
Canon assures us that any future firm- 
ware updates issued for the Canon 60D 
will also work on the 60Da. 

A feature I was hoping for in the 60Da 
is one offered only in the 5D series — the 
ability to buffer up to five images in the 
camera and then take a single dark frame 
that's applied to all of them. This speeds 
up deep-sky shooting immensely. As it is, 
for those of us who prefer to use "real- 
time" dark frames, the 60Da requires us to 
alternate light and dark frames, doubling 
the time it takes to acquire a set of images. 

At a suggested retail price of $1,500, 
the Canon 60Da carries about a $500 
premium over the street price of a 60D. 
But this increase is mitigated somewhat 
by the worthwhile accessories included 
with the 60Da. A modified 60D purchased 
from a third party costs upwards of $1,400, 
but it won't include the accessories, and, 
as mentioned earlier, will be compromised 
for daytime photography. 

With all that in mind, the Canon 60Da 
is a great deal. There is certainly no ques- 
tion in my mind that the 60Da is the best 
choice for anyone wanting a DSLR for all 
types of astrophotography, as well as for 
daytime use. Many thanks go to Canon for 
serving our astronomy market. + 

Sky & Telescope contributing editor Alan 
Dyer has been using DSLRsfor astrophotog- 
raphy since 2004. Galleries of his images are 
at www.amazingsky.photoshelter.com and 
at his photo blog www.amazingsky.net. 



As explained in the text, the 60Da is a respectable camera for daytime photography without the addition of special filters or major color correction dur- 
ing image processing. 




Canon 7D — The stock 7D set on Auto White 
Balance provides the expected color balance 
with blue sky, green grass, and neutral grays on 
the weathered farm buildings. 



Canon 60Da — Set for a Auto White Balance (center frame), the 60Da is close to the 7D but with 
a slightly redder tint (the grass is not as green, the buildings are a warmer tone). Switching to a 
Custom White Balance (right), which is explained in the camera manual, helps compensate for the 
warmer tone, producing a daylight shot with better overall color balance. 



SkyandTelescope.com September 2012 41 



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OBSERVING 

September 2012 




In This Section 



44 Sky at a Glance 

44 Northern Hemisphere Sky Chart 

45 Binocular Highlight: Scutum Odd Couple 

46 Planetary Almanac 

47 Northern Hemisphere's Sky: 
The Celestial Dolphin 

48 Sun, Moon & Planets: 

Planets Fly Low at Dusk and High at Dawn 

The Hubble Space Telescope caught Ganymede, Jupiter's 
largest moon, disappearing behind the planet in April 2007. 

PHOTOGRAPH: 

NASA / ESA / Erich Karkoschka / University of Arizona 



50 Celestial Calendar 

50 Spotting Uranus and Neptune 

51 Action at Jupiter 

51 Unusual Occultations 

51 Minima of Algol 

52 Jupiter's Moons 

53 So, Where Are the Wild Ducks?! 

54 Exploring the Solar System: 
A Last Hurrah at Uranus 

58 Deep-Sky Wonders: Guide Me, Cygnus 

60 Web Links: Star Parties and Astro Events 

Additional Observing Stories: 

63 Going Deep: The Gamma Cygni Nebula 



SkyandTelescope.com September 2012 43 



OBSERVING 

Sky at a Glance 



SEPTEMBER 2012 

8 DAWN: Jupiter is about 1° above the 3rd-quarter 
Moon for North Americans; see page 48. 

12 DAWN: Venus is left of the waning crescent Moon. 

13-14 EVENING OR NIGHT: Algol is at minimum 
brightness for roughly 2 hours centered on 
10:02 p.m. on the 13th PDT (1:02 a.m. on the 
14th EDT); see page 51. 

14 DAWN: A very thin crescent Moon should be 

visible low in the east starting about an hour before 
sunrise. Look lower right of lst-magnitude Regulus. 

14-28 DAWN: The zodiacal light is visible in the east 
120 to 80 minutes before sunrise from dark 
locations at mid-northern latitudes. Look for a tall, 
broad, rightward-leaning pyramid of light with 
Venus directly on its axis. 

16 EVENING: Algol is at minimum brightness for 
roughly 2 hours centered on 9:51 p.m. EDT. 

18, 19 DUSK: Low in the west-southwest, the waxing 

crescent Moon is well to Saturn's left on September 
18th and close to the left of Mars on the 19th; see 
page 48. 

22 AUTUMN BEGINS in the Northern Hemisphere 
at the equinox, 10:49 a.m. EDT. 

22, 23 ALL NIGHT: Binoculars and telescopes show that 
Uranus is extremely close to the almost identically 
bright star 44 Piscium; see page 50. 

28-29 ALL NIGHT: Uranus is at opposition to the Sun, 

rising around sunset, soaring highest in the middle 
of the night, and setting around sunrise. 



Planet Visibility 



Mercury 



Hidden in the Sun's glow all month. 




PLANET VISIBILITY SHOWN FOR LATITUDE 40° NORTH AT MID-MONTH 



Moon Phases 



SUN | MON I TUE I WED I THU I FRI | SAT 



• • <• s « «« ' 

10 d 11 1 12 r r 

r )■ )")">i"» 



J|( 



( ( c : ( 



Go out within an hour of a time 
listed to the right. Turn the map 
around so the yellow label for the 
direction you're facing is at the 
bottom. That's the horizon. Abo 
it are the constellations ii 
you. The center of the m 
overhead. Ignore th^ nar 
of the map above h 
you're not fac 1 ™ 

EXACT FOR LA. 
40° NORTH. 



» 




Galaxy 

Double star 

Variable star 

Open cluster 

Diffuse nebula 

Globular cluster 

Planetary nebula M 



iupej 




Late July Midnight- 
Early Aug 11 p.m.* 
Late Aug 10 p.m.* 
Early Sept 9 p.m.* 
Late Sept Dusk 

* Daylight-saving time. 








Binocular Highlight 



Scutum Odd Couple 

Despite its modest size, Scutum is a binocular wonderland 
rich with intriguing Milky Way fields and home to a pair of 
Messier clusters. And to borrow the setup of an old joke, 
one cluster is among the most pleasing and rich in the sky 
— and the other is M26. 

Mil and M26 make an interesting open cluster odd 
couple. Let's begin with the more eye-catching of the 
two: Mil, otherwise known as the Wild Duck Cluster. 
This grouping got its distinctive name from Admiral W. 
H. Smyth who, in his 19th-century guidebook, likened its 
appearance to "a flight of wild ducks" (see page 53). Listed 
as a 5.8-magnitude object, Mil is a snap to sweep up in any 
pair of binoculars. In my 10x50s, I see a single, lead duck 
situated near the southeast corner of a little hazy cloud 
of unresolved starlight. Beyond the southeast edge of the 
cluster lies another star — a sort of stray duck. What I find 
intriguing is that the stray shines at magnitude 9.1, while 
the lead duck is 8.6, yet the former actually appears brighter 
to me. Does it look that way to you too? Perhaps some kind 
of contrast effect is in play here. 

As bright and easy as Mil is, M26 is dim and difficult. 
Indeed, this 8th-magnitude object holds the distinction 
of being the most elusive open cluster in the Messier 
catalog. Luckily, its position is easy to pin down — M26 
completes a right triangle with the nearby 5th-magnitude 
stars Epsilon (e) and Delta (5) Scuti. In my 10x30 image- 
stabilized binoculars, the cluster looks like a slightly 
soft, 8th-magnitude dot. My 10x50s reveal a single 
9.2-magnitude star embedded within M26, giving the 
cluster the appearance of a miniature, mirror- reverse 
version of neighboring Mil. ♦ — Gary Seronik 



AQUILA 



*& 



ri a n gl e \ 5 
M26 ' : '" 




5 Star 

4 magnitudes 



SCUTUM 



Watch a SPECIAL VIDEO 




To watch a video tutorial on how to use 
the big sky map on the left, hosted by 
S&T senior editor Alan MacRobert, visit 



SkyandTelescope.com/maptutorial. 
SkyandTelescope.com September 2012 



A 



OBSERVING 



Planetary Almanac 



Mercury 



Sepl 11 



-^ lfi 30 



Sun ai 


id Planets, Septem 

Sept Right Ascension Declination 


ber 2012 

Elongation Magnitude 


Diameter 


Illumination 


Distance 


Sun 


1 


10 h 41.6 m 


+8° 16' 


— 


-26.8 


31' 42" 


— 


1.009 


30 


12 h 25.9 m 


-2° 48' 


— 


-26.8 


31' 57" 


— 


1.001 


Mercury 


1 


10 h 09.3 m 


+13° 10' 


9° Mo 


-1.4 


5.3" 


93% 


1.273 


11 


ll h 21.8 m 


+5° 52' 


2°Ev 


-1.8 


4.9" 


100% 


1.379 


21 


12 h 26.3 m 


-2° 00' 


9°Ev 


-0.9 


4.8" 


97% 


1.392 


30 


13 h 19.2 m 


-8° 38' 


14° Ev 


-0.4 


5.0" 


92% 


1.353 


Venus 


1 


7 h 41.3 m 


+19° 21' 


45° Mo 


-4.3 


19.9" 


58% 


0.836 


11 


8 h 26.5 m 


+17° 55' 


44° Mo 


-4.2 


18.3" 


63% 


0.912 


21 


9 h u .2™ 


+15° 40' 


43° Mo 


-4.2 


16.9" 


67% 


0.985 


30 


9 h 53.2 m 


+12° 59' 


41° Mo 


-4.1 


15.9" 


70% 


1.049 


Mars 


1 


14 h 11. m 


-13° 45' 


56° Ev 


+ 1.2 


5.2" 


91% 


1.803 


16 


14 h 50.1 m 


-17° 03' 


52° Ev 


+ 1.2 


5.0" 


92% 


1.876 


30 


15 h 29.0 m 


-19° 46' 


48° Ev 


+ 1.2 


4.8" 


93% 


1.937 


Jupiter 


1 


4 h 52.7 m 


+21° 44' 


84° Mo 


-2.3 


39.2" 


99% 


5.025 


30 


5 h 00.3 m 


+21° 54' 


111° Mo 


-2.5 


42.9" 


99% 


4.591 


Saturn 


1 


13 h 40.1 m 


-7° 55' 


47° Ev 


+0.8 


16.0" 


100% 


10.416 


30 


13 h 51.6 m 


-9° 04' 


22° Ev 


+0.7 


15.6" 


100% 


10.688 


Uranus 


16 


h 26.5 m 


+2° 03' 


166° Mo 


+5.7 


3.7" 


100% 


19.085 


Neptune 


16 


22 h 12.9 m 


-11°43' 


158° Ev 


+7.8 


2.3" 


100% 


29.062 


Pluto 


16 


18 h 28.7 m 


-19° 38' 


103° Ev 


+ 14.1 


0.1" 


100% 


32.052 



Neptune 



The table above gives each object's right ascension and declination (equinox 2000.0) at h Universal Time on selected 
dates, and its elongation from the Sun in the morning (Mo) or evening (Ev) sky. Next are the visual magnitude and 
equatorial diameter. (Saturn's ring extent is 2.27 times its equatorial diameter.) Last are the percentage of a planet's disk 
illuminated by the Sun and the distance from Earth in astronomical units. (Based on the mean Earth-Sun distance, 1 a.u. 
is 149,597,871 kilometers, or 92,955,807 international miles.) For other dates, see SkyandTelescope.com/almanac. 

Planet disks at left have south up, to match the view in many telescopes. Blue ticks indicate the pole currently tilted 
toward Earth. 




The Sun and planets are positioned for mid-September; the colored arrows show the motion of each during the month. The Moon is plotted for evening dates in the Americas when it's waxing 
(right side illuminated) or full, and for morning dates when it's waning (left side). "Local time of transit" tells when (in Local Mean Time) objects cross the meridian — that is, when they appear due 
south and at their highest — at mid-month. Transits occur an hour later on the 1st, and an hour earlier at month's end. 



^46 






46 September 2012 SKY & TELESCOPE 



OBSERVING 



Northern Hemisphere's Sky 



Fred Schaaf welcomes your 
comments atfschaaf@aol.com. 



Fred Schaaf 



-i 



The Celestial Dolphin 

This tiny but shapely constellation is full of wonder. 



For the past two months in this column we toured 
entire classes of celestial objects, objects scattered far 
and wide across the summer sky. This month I want to 
concentrate on a tiny part of the September evening sky: 
the minute pattern of Delphinus the Dolphin. We will 
find that despite its relatively few stars, this little constel- 
lation has a surprisingly large amount of legend, lore, and 
observational interest. 

Delphinus in context. Delphinus is charming partly 
due to the way it fits into the larger scene of the summer 
sky. Its attractive position in the big picture is undeni- 
able — just outside the region enclosed by the Summer 
Triangle, which contains the even smaller pattern of 
Delphinus's neighboring constellation Sagitta the Arrow. 
But Sagitta is fairly dim, requiring a rather dark sky to 
appreciate properly. Delphinus is much brighter — easy 
to spot with the naked eye on a clear night in a small city 
or suburb. 

Delphinus also lies against a darker natural back- 
ground than Sagitta because it's just outside the main 
band of the summer Milky Way. The Milky Way is often 
imagined to be a river, so maybe this is a fresh-water 
dolphin. Or perhaps it's leaping out of the celestial sea 
that spills out of Aquarius's Water Jug — a region whose 
watery constellations now flood most of the southeast sky. 

The head of Pegasus (which includes 2nd-magnitude 
Enif, the Nose) also points to Delphinus. Even more 
interestingly, Delphinus lies along two lines of wonder- 
ful sights for binoculars and telescopes. One consists of 
increasingly small star patterns: little Delphinus, smaller 
Sagitta, and smallest Brocchi's Cluster (better known to 
amazed binocular and telescope users as the Coathanger). 
The other line runs from the Ring Nebula (M57) through 
the great color- contrast double star Albireo and the Dumb- 
bell Nebula (M27). 

Delphinus's best deep-sky object. The diamond of 
Delphinus includes its own telescopic gem, whose beauty 
is detectable in small telescopes. I'm referring to Gamma 
(y) Delphini, which marks the nose of the Dolphin. Its 
4.5- and 5. 5 -magnitude components are separated by 10", 
and its colors are usually held to be gold and green. Can 
you see the emerald tint of the secondary? Just 15' to the 
southwest is James Mullaney's "Ghost Double" — Struve 
2725 (E2725), whose 6"-wide pair of 7.6- and 8.4-magni- 
tude stars is like a ghost image of much brighter Gamma. 




Delphinus in legend and lore. The pattern of Delphi- 
nus, a diamond with a tail, really does suggest a dolphin. 
The most famous dolphin in Greek mythology, and the 
one most often associated with this constellation, is the 
one that saved the poet Arion when he leaped into the 
sea to escape being murdered by the piratical crew of the 
boat he was sailing on. In the middle of the 20th century, 
research began to hint that dolphins might be even more 
intelligent than previously thought, possessing brains 
that are extraordinarily complex, and also quite large for 
animals of their size. So it isn't surprising that the first 
group of scientists gathered to discuss searching for extra- 
terrestrial intelligence — a group that included astrono- 
mers Frank Drake and Carl Sagan — decided to call itself 
the Order of the Dolphin. 

There's a final amusing piece of lore about Delphinus. 
Let me give you a hint. It involves an astronomer called 
Nick Hunter. Never heard of him? You can find him in 
many histories of astronomy but not under quite that 
name. How many of you know who he was? I'll identify 
him and his marvelous connection to Delphinus here 
next month. ♦ 

SkyandTelescope.com September 2012 



A 



OBSERVING 



Sun, Moon & Planets 



Planets Fly Low at Dusk and High 



Venus and Jupiter dominate the predawn sky. 



At dusk in September, Mars and Saturn 
glimmer low in the southwest to west- 
southwest. Dim Uranus is visible all night 
long and has a very special conjunction. 

Brilliant Jupiter rises in the late evening 
and shines at its highest in the south by 
dawn. Even-brighter Venus comes up long 
before the Sun and is about 1/3 the way up 
the eastern sky by morning twilight. 

But the most impressive solar system 
events to observe in the Americas this 
month may be three impressive Moon- 
planet conjunctions. 

DUSK 

Mars and Saturn both start September 
a little more than 10° high in the west- 
southwest at mid-twilight for skywatchers 
at mid-northern latitudes. Look for them 
well to the lower left of brighter Arcturus. 
The two planets are 10° apart in eastern 
Virgo on September 1st (Saturn is the one 
on the right), but by month's end Mars 
has moved all the way into Libra, leaving 
Saturn far behind. 

Mars nearly keeps pace with the Sun in 
its eastward motion against the stars, set- 
ting about 2 hours after sunset for all the 
rest of this year! Saturn, on the other hand, 



is on the way out. It's very low in bright twi- 
light by the end of September and will soon 
be lost from view. Spica, 5° or 6° below 
Saturn, is probably too low to spot by late 
September. Mars shines at magnitude +1.2 
all September, Saturn around +0.8. 

Use binoculars on September 14-16 
to find the wide double star Alpha Librae 
(Zubenelgenubi) about 1° from Mars. The 
star's 2.7- and 5.2-magnitude components 
are almost 4' apart, making them an easy 
split in binoculars. 

Mercury is too close to the Sun all 
month to observe without a telescope. 

EVENING TO DAWN 

Uranus is at opposition on the night 
of September 28-29, so it rises around 
sunset and is highest in the middle of the 
night. Uranus shines at magnitude 5.7 
all September, bright enough to see with 
the naked eye in very dark clear skies. Its 
bluish (some see greenish) disk appears 
3.7" wide in a telescope. Finder charts for 
Uranus and Neptune are on page 50. They 
show that Uranus retrogrades from north- 
western Cetus into Pisces this month. 
What's really exciting is that on the Ameri- 
can evenings of September 22nd and 23rd, 



^48 



Moon 




Dusk, Sept 18 -20 

45 minutes after sunset 


Sept 20 v . 






Antares 

• • 


Moon 

Sept 19 . Mars 






Moon 
Sept 18 


Saturn 

• 




^P^^^^ 


Spica 

1 ^P^^^H 




Looking Southwest 


^\ 



Uranus lies little more than 1' from almost 
identically bright 44 Piscium. 

Neptune is two magnitudes dimmer 
than Uranus and, in Aquarius, crosses the 
meridian more than two hours earlier. 

Pluto is more than eight magnitudes 
dimmer than Uranus and is highest in 
the early evening; see page 52 of the June 
issue for a finder chart. 

Jupiter rises a little before midnight 
(daylight-saving time) as September 
opens, and around 10 p.m. as the month 
closes. The enormous planet brightens 
from -2.3 to -2.5 and grows from 39" to 
43" wide this month. Jupiter is more than 
2 /3 of the way up the southern sky when it 
transits the meridian around dawn. 

Jupiter is at quadrature (90° west of 
the Sun) on September 7th, so this is the 
month when the planet and its Galilean 
satellites cast their shadows farthest to the 



Dawn, Sept 7-9 

1 hour before sunrise 




ORION 



Looking Southeast, High in the Sky 



September 2012 SKY & TELESCOPE 



To see what the sky looks like at any given time and date, go to SkyandTelescope.com/skychart. 



Fred Schaaf 




at Dawn 



side. Jupiter's eastward motion away from 
Aldebaran is slowing and will soon reverse. 

Venus has yet another superb month for 
viewers at mid-northern latitudes. We see 
it rise about 3V2 hours before the Sun all 
month, shining at magnitude -4.2 about Vi 
of the way up the eastern sky by the middle 
of morning twilight. Venus now is past 
greatest elongation, so its phase is more 
than half lit, and its apparent diameter 
shrinks from 20" to 16" during September. 

But the planet speeds across much 
scenic starry background this month. On 
September 6th, Venus is in line with Castor 
and Pollux (though IOV2 from Pollux). On 
September 12th and 13th Venus passes 
about 2V2° south of M44, the Beehive Star 
Cluster — though the crescent Moon 
nearby interferes with the naked- eye view. 
On September 14th Venus is a little more 
than 1° from Delta Cancri (Asellus Austra- 



Dawn, Sept 12-14 

1 hour before sunrise 



Regulus 



Looking East 




lis). On September 23rd Venus crosses into 
Leo, ending the month just 3° upper right 
of Regulus — which it will meet in a shock- 
ingly close conjunction on October 3rd. 

MOON AND SUN 

The Moon is almost precisely at last quar- 
ter at dawn on September 8th, when North 




ORBITS OF THE PLANETS 

The curved arrows show each planet's movement 
during September. The outer planets don't change 
position enough in a month to notice at this scale. 

Americans see bright Jupiter perched 
dramatically close just above it. The Moon 
occults Jupiter shortly after daybreak on 
the 8th for most of South America. 

The waning lunar crescent is not far to 
the right of Venus on September 12th, and 
the waxing crescent is very close to Mars's 
left at dusk on September 19th. The Moon 
occults Mars shortly before or after sunset 
on the 19th for much of South America. 

The Sun reaches the September 
equinox at 10:49 a.m. EDT on Septem- 
ber 22nd, starting autumn in the North 
Hemisphere and spring in the Southern 
Hemisphere. The Harvest Moon — the 
full Moon closest to the autumn equi- 
nox — occurs this year on the American 
evening of September 29th. + 

These scenes are drawn for near the middle of 
North America (latitude 40° north, longitude 
90° west); European observers should move 
each Moon symbol a quarter of the way toward 
the one for the previous date. In the Far East, 
move the Moon halfway. The blue 10° scale bar 
is about the width of your fist at arm's length. 
For clarity, the Moon is shown three times its 



actual apparent size. 

SkyandTelescope.com September 2012 



49 ^m 



OBSERVING 

Celestial Calendar 



Spotting Uranus and Neptune 

The twin ice giants have taken on more cosmic importance than we expected. 



Anyone with binoculars can spot the two 
outermost major planets: Uranus easily 
at magnitude 5.7, Neptune with more dif- 
ficulty at 7.8. Almost any telescope at fairly 
high power will show them to be, if not 
exactly distinct balls, at least nonstellar in 
appearance; they're 3.7 and 2.3 arcseconds 
wide this season. 

Uranus and Neptune are currently 
south of the Great Square of Pegasus, near 
the Circlet of Pisces and under the elbow 
of Aquarius respectively. They're well up 
by late evening in August and September. 
The finder charts below show their posi- 
tions among the stars for the rest of this 
observing season. The large chart is all 
you may need for Uranus; it shows stars to 
as faint as magnitude 6.5. The small black 
boxes there show the areas covered by the 

The near-twin outermost planets creep 
westward for the next few months, then double 
back on themselves. The black boxes below 
show the areas of the deeper charts at right. 



close-ups on the right, where stars are 
plotted to magnitude 9.5. 

Uranus appulse. Set your calendar 
to remind you to look at Uranus on the 
evenings of September 21st through 23rd. 
It will be passing close by the almost 
identically bright star 44 Piscium. They'll 
be about 3.3 arcminutes apart on the 
American evening of the 21st, 1.0' on the 
22nd, and 1.7' on the 23rd. (Their closest 
approach, or appulse, is 0.7' around 12 h 
September 23rd UT, good viewing for 
Australia.) Look for subtle color contrast 
between the yellow star, spectral type G5 
III, and slightly aquamarine-gray Uranus. 

Uranus comes to opposition on 
September 29th. Neptune was at opposi- 
tion on August 24th. The story of how 
Uranus's rotation period was determined 
visually by an amateur (in fact a former 
S&T editor) is told in the article that 
begins on page 54. 

Neptunes everywhere. Only recently 
have astronomers realized the wider sig- 



nificance of Uranus and Neptune in the 
universe at large. Rather than being unique 
oddballs as we used to think of them (with 
14.5 and 17.1 Earth masses, respectively), 
they've turned out to be the most common 
type of planet in the universe discovered 
so far. Of the 2,321 likely extrasolar planets 
that NASA's Kepler project has announced 
to date, "Neptunes" (broadly defined) 
amount to 1,118, whereas Jupiters and 
super- Jupiters add up to only 281. 

Super- Earths, with 2 to 10 Earth 
masses, put in an impressive showing 
with 676 candidates, and these likely 
outnumber Neptunes in reality consider- 
ing that small planets are harder to detect. 
Smaller Earth analogs may be even more 
abundant; Kepler has logged 246 candi- 
dates with less than 1.25 Earth diameters. 
Theorists expect the trend of "the smaller 
the more" to continue on down, but the 
statistics for small, hard-to- detect plan- 
ets are still fairly weak. For the moment, 
Uranus-Neptune analogs rule. 



k 




•1 

-+3° 
_ O h 35™ 


1 

h 30 m 

• 


1 1 
h 25 m # h *20 m h 


'15 m . -1 


-+2° 


Augl ^\^ 
Sep 1 ^ 


1 t>a*i PISCES 
4 44 "'W 




!«. 

W> 7 # 


C 


ETUS 


~>\/ Febl . * 
Octl.. ^^C* janl, 


• 


re / • 
! 8- 

CO 

X 9 • 


, TO 




'• NovV ^> 


1 zd 


— b \ • 


1 


! , • , Decl 


1 





1 - 


1 




• 1 


■ 


— ll° 






• 




-§ 5 • 

M 6 • 

c 


--12° 


Augl 


Sepl 

40 


janl 
Octl 


#38 ' • 
Decl 

Novl 


8? 7 . 
! 8 • 

CO 

X 9 • 


— 






AQUARIUS 








. 


1 




22 h 20 m 

1 •^ 


22 h 15 m • 
1 




22 h 1Q m 
1 







September 2012 SKY & TELESCOPE 



WKKIMHIKMIKMIK^^ 



Action at Jupiter 



Alan MacRobert 



? 



After lighting the morning sky all sum- 
mer, Jupiter rises in mid- evening in Sep- 
tember (in Taurus) and shines moderately 
well up in the east by midnight or 1 a.m. 
daylight-saving time. 

Even the smallest scope shows Jupi- 
ter's four large Galilean moons. Binocu- 
lars usually show at least two or three of 
them. Identify them with the diagram 
on the next page. Also listed there are all 
their interactions with Jupiter's disk and 
shadow during September, events fasci- 
nating to watch. 

And here are the times, in Universal 
Time, when Jupiter's Great Red Spot 
should cross the planet's central merid- 
ian, the imaginary line down the center of 
Jupiter from pole to pole. The dates (also 
in UT) are in bold. Eastern Daylight Time 
is UT minus 4 hours; Pacific Daylight 
Time is UT minus 7 hours. The Red Spot 
appears closer to Jupiter's central merid- 
ian than to the limb for 50 minutes before 
and after these times: 

September 1, 9:51, 19:47; 2, 5:43, 15:38; 
3, 1:34, 11:30, 21:25; 4, 7:21, 17:17; 5, 3:12, 
13:08, 23:04; 6, 9:00, 18:55; 7, 4:51, 14:47; 



8, 0:42, 10:38, 20:34; 9, 6:29, 16:25; 10, 
2:21, 12:16, 22:12; 11, 8:08, 18:03; 12, 3:59, 
13:55, 23:50; 13, 9:46, 19:42; 14, 5:37, 15:33; 
15, 1:29, 11:24, 21:20; 16, 7:16, 17:11; 17, 
3:07, 13:03, 22:58; 18, 8:54, 18:50; 19, 4:45, 
14:41; 20, 0:37, 10:32, 20:28; 21, 6:24, 16:19; 
22, 2:15, 12:11, 22:06; 23, 8:02, 17:58; 24, 
3:53,13:49,23:45; 25,9:40,19:36; 26,5:32, 
15:27; 27, 1:23, 11:18, 21:14; 28, 7:10, 17:05; 
29, 3:01, 12:57, 22:52; 30, 8:48, 18:44. 

These times assume that the spot is 
centered at System II longitude 184°. If 
it has moved elsewhere, it will transit 
l 2 /3 minutes late for every 1° of longitude 
greater than 184°, or IVi minutes early for 
every 1° less than 184°. 

Markings on Jupiter appear a little 
more contrasty through a blue or green 
filter. The larger your scope, the darker 
the blue or green can be; you want enough 
light to see details clearly, but not so much 
that you lose visual details to glare. 

Try several magnifications to find the 
one that shows the most in the current 
seeing conditions. And keep looking. 
More and more details come out with 
protracted scrutiny. 




Minima of Algol 



Algol, the prototype eclipsing variable star, fades 
from its usual magnitude 2.1 to 3.4 and back every 
2.87 days. It stays near minimum light for two hours, 
and it takes several additional hours to fade and to 
rebrighten. Shown above are magnitudes of com- 
parison stars (with the decimal points omitted). 
The "21" star is the bright foot of Andromeda. 

These geocentric time predictions (times as seen from the moving Earth) are from the heliocentric (Sun-centered) 
elements Min. = JD 2452253.559 + 2.867362E, where f is any integer. Courtesy Gerry Samolyk (AAVSO). 



Aug. 


UT 


2 


4:53 


5 


1:41 


7 


22:30 


10 


19:19 


13 


16:07 


16 


12:56 


19 


9:45 


22 


6:33 


25 


3:22 


28 


0:11 


30 


20:59 



Sept. 


UT 


2 


17:48 


5 


14:36 


8 


11:25 


11 


8:14 


14 


5:02 


17 


1:51 


19 


22:40 


22 


19:28 


25 


16:17 


28 


13:06 



Unusual 
Occultations 

September brings four occultations that 
are unusual in various ways and challeng- 
ing in different ways. 

• The naked-eye star Alpha 2 Librae, 
magnitude 2.7, will be occulted for up 
to 2.6 seconds by the faint asteroid 363 
Padua on September 16th along a path 
from Washington state to Florida — in 
the daytime! If the blue sky is especially 
clear and clean you might be able to see 
the star in a telescope, especially at high 
power, if you can get aimed exactly right. 
Alpha 2 is the brighter star of the wide 
Alpha Librae pair. But can you hold it in 
continuous view well enough to tell an 
occultation from an atmospheric shim- 
mer or an eye twitch? Some observers will 
surely try. For details see the link below. 

• Far beyond the asteroid belt, the 
trans-Neptunian object 2000 PD 30 may 
occult an 8.0-magnitude star in Aquarius 
for up to 6 seconds somewhere in the 
Americas, sometime around 6:38 Septem- 
ber 20th Universal Time. . . plus or minus 
an hour or more! Objects that far away 
cast very uncertain occultation tracks, 
but timings of these events are especially 
desirable because such remote objects 
are otherwise hard to study. Check the 
link below for any late improvements to 
the prediction. If enough observers all 
across the continent keep watch, one 

or more may get lucky. This cold outer 
object is estimated to be 80 miles (125 
km) across. 

• The Moon occults Jupiter on Sep- 
tember 8th — but only for central and 
southern South America in the daytime 
(around sunrise on the west coast). 

• The Moon occults Mars on Septem- 
ber 19th — but again only for central and 
southern South America, and partly in 
daytime (in evening twilight for some of 
the Atlantic coast). 

Further information on these events: 
skypub.com/sept2012occultations. 






A 



OBSERVING 

lestial Calend; 



Jupiter's Moons 



Phenomena of Jupiter's Moons, September 2012 



Sept. 1 


3:31 


.Oc.R : 




21:12 


.Sh.l j 




22:32 


Tr.l j 




23:16 


I.Sh.l : 




23:21 


.Sh.E j 


Sept. 2 


0:41 


Tr.E i 




1:38 


I.Sh.E j 




2:02 


I.Tr.l j 




4:23 


I.Tr.E j 




18:27 


.Ec.D j 




22:00 


.Oc.R j 


Sept. 3 


15:41 


.Sh.l j 




17:01 


Tr.l j 




17:43 


I.Ec.D j 




17:49 


.Sh.E : 




19:09 


Tr.E j 




20:05 


I.Ec.R : 




20:25 


I.Oc.D j 




22:44 


II. Sh.l j 




22:47 


I.Oc.R j 


Sept. 4 


0:38 


II. Sh.E j 




4:15 


1 I.Tr.l j 




6:06 


ll.Tr.E j 




12:55 


.Ec.D j 




16:28 


.Oc.R j 


Sept. 5 


10:09 


.Sh.l j 




11:29 


Tr.l j 




12:17 


.Sh.E ! 




12:34 


I.Sh.l i 




13:37 


Tr.E j 




14:56 


I.Sh.E i 




15:21 


I.Tr.l j 




17:41 


I.Tr.E : 


Sept. 6 


7:24 


.Ec.D j 




10:57 


.Oc.R ; 


Sept. 7 


4:38 


.Sh.l j 




5:57 


Tr.l ! 




6:46 


.Sh.E : 




7:00 


I.Ec.D j 




8:06 


Tr.E : 




9:23 


I.Ec.R j 




9:43 


I.Oc.D j 




12:04 


I.Oc.R j 




12:34 


II. Ec.D j 




14:30 


II.Ec.R j 




18:09 


II.Oc.DJ 




20:01 


II.Oc.R i 


Sept. 8 


1:52 


.Ec.D j 




5:25 


.Oc.R : 




23:06 


.Sh.l j 



Sept. 


9 


0:26 


Tr.l 






1:14 


.Sh.E 






1:53 


I.Sh.l 






2:34 


Tr.E 






4:15 


I.Sh.E 






4:39 


I.Tr.l 






7:00 


I.Tr.E 






20:21 


.Ec.D 






23:54 


.Oc.R 


Sept. 





17:34 


.Sh.l 






18:54 


Tr.l 






19:43 


.Sh.E 






20:17 


I.Ec.D 






21:02 


Tr.E 






22:40 


I.Ec.R 






22:59 


I.Oc.D 


Sept. 




1:20 


I.Oc.R 






2:43 


II. Sh.l 






4:38 


II. Sh.E 






8:14 


1 I.Tr.l 






10:04 


ll.Tr.E 






14:49 


.Ec.D 






18:22 


.Oc.R 


Sept. 


12 


12:03 


.Sh.l 






13:22 


Tr.l 






14:11 


.Sh.E 






15:11 


I.Sh.l 






15:31 


Tr.E 






17:33 


I.Sh.E 






17:57 


I.Tr.l 






20:17 


I.Tr.E 


Sept. 




9:18 


.Ec.D 






12:51 


.Oc.R 


Sept. 




6:31 


.Sh.l 






7:50 


Tr.l 






8:39 


.Sh.E 






9:34 


I.Ec.D 






9:59 


Tr.E 






11:57 


I.Ec.R 






12:16 


I.Oc.D 






14:36 


I.Oc.R 






16:34 


II. Ec.D 






18:30 


II.Ec.R 






22:06 


II.Oc.D 






23:58 


II.Oc.R 


Sept. 


15 


3:46 


.Ec.D j 






7:19 


.Oc.R 


Sept. 


6 


0:59 


.Sh.l 






2:18 


Tr.l 






3:08 


.Sh.E 







4:27 


Tr.E ; 






4:30 


I.Sh.l ! 






6:52 


I.Sh.E ; 






7:14 


I.Tr.l ! 






9:34 


I.Tr.E : 






22:15 


.Ec.D j 


Sept. 


17 


1:47 


.Oc.R ; 






19:28 


.Sh.l j 






20:46 


Tr.l ; 






21:36 


.Sh.E ; 






22:51 


I.Ec.D : 






22:55 


Tr.E j 


Sept. 


18 


1:14 


I.Ec.R ! 






1:31 


I.Oc.D ; 






3:52 


I.Oc.R ; 






6:43 


II. Sh.l : 






8:38 


II. Sh.E ; 






12:09 


1 I.Tr.l : 






13:59 


ll.Tr.E j 






16:43 


.Ec.D ; 






20:15 


.Oc.R ; 


Sept. 


19 


13:56 


.Sh.l ; 






15:14 


Tr.l : 






16:05 


.Sh.E j 






17:23 


Tr.E ; 






17:48 


I.Sh.l ; 






20:10 


I.Sh.E : 






20:30 


I.Tr.l j 






22:50 


I.Tr.E j 


Sept. 


20 


11:12 


.Ec.D j 






14:43 


.Oc.R ; 


Sept. 


2" 


8:24 


.Sh.l j 






9:42 


Tr.l ! 






10:33 


.Sh.E ; 






11:51 


Tr.E j 






12:08 


I.Ec.D ; 






14:31 


I.Ec.R ; 






14:46 


I.Oc.D : 






17:07 


I.Oc.R ; 






20:33 


II. Ec.D ; 






22:31 


II.Ec.R j 


Sept. 


22 


1:58 


II.Oc.D: 






3:50 


II.Oc.R; 






5:40 


.Ec.D ; 






9:11 


.Oc.R ; 


Sept. 


23 


2:53 


.Sh.l j 






4:10 


Tr.l ; 






5:01 


.Sh.E ; 






6:18 


Tr.E : 






7:06 


I.Sh.l j 







9:29 


I.Sh.E 






9:47 


I.Tr.l 






12:07 


I.Tr.E 


Sept. 


24 


0:09 


.Ec.D 






3:39 


.Oc.R 






21:21 


.Sh.l 






22:38 


Tr.l 






23:30 


.Sh.E 


Sept. 


25 


0:46 


Tr.E 






1:25 


I.Ec.D 






3:48 


I.Ec.R 






4:01 


I.Oc.D 






6:22 


I.Oc.R 






10:42 


II. Sh.l 






12:39 


II. Sh.E 






15:59 


1 I.Tr.l 






17:49 


ll.Tr.E 






18:37 


.Ec.D 






22:06 


.Oc.R 


Sept. 


26 


15:49 


.Sh.l 




17:05 


Tr.l 




17:58 


.Sh.E 




19:14 


Tr.E 






20:24 


I.Sh.l 






22:47 


I.Sh.E 






23:02 


I.Tr.l 


Sept. 


27 


1:22 


I.Tr.E 






13:06 


.Ec.D 






16:34 


.Oc.R 


Sept. 


28 


10:18 


.Sh.l 






11:33 


Tr.l 




12:26 


.Sh.E 






13:41 


Tr.E 






14:42 


I.Ec.D 




17:05 


I.Ec.R 






17:15 


I.Oc.D 






19:36 


I.Oc.R 


Sept. 


29 


0:32 


II. Ec.D 






2:31 


II.Ec.R 






5:46 


II.Oc.D 






7:34 


.Ec.D 






7:37 


II.Oc.R 






11:02 


.Oc.R 


Sept. 


30 


4:46 


.Sh.l 






6:00 


Tr.l 






6:55 


.Sh.E 






8:09 


Tr.E 






9:43 


I.Sh.l 






12:06 


I.Sh.E 






12:17 


I.Tr.l 






14:37 


I.Tr.E 



The wavy lines represent Jupiter's four big satellites. The central 
vertical band is Jupiter itself. Each gray or black horizontal band is 
one day, from h (upper edge of band) to 24 h UT (GMT). UT dates are 
at left. Slide a paper's edge down to your date and time, and read 
across to see the satellites' positions east or west of Jupiter. 



Every day, interesting events happen between Jupiter's satellites and the planet's disk or shadow. The first columns give the date 
and mid-time of the event, in Universal Time (which is 4 hours ahead of Eastern Daylight Time). Next is the satellite involved: 
I for lo, II Europa, III Ganymede, or IV Callisto. Next is the type of event: Oc for an occultation of the satellite behind Jupiter's 
limb, Ec for an eclipse by Jupiter's shadow, Tr for a transit across the planet's face, or Sh for the satellite casting its own shadow 
onto Jupiter. An occultation or eclipse begins when the satellite disappears (D) and ends when it reappears (R). A transit or 
shadow passage begins at ingress (I) and ends at egress (E). Each event is gradual, taking up to several minutes. Predictions 
courtesy IMCCE / Paris Observatory. 



Deep-sky tip: Photos usually make bright and faint stars look more alike than the eye sees them. 



So, Where Are the 



Ducks?! 



A favorite deep-sky sight of late sum- 
mer and early fall is Mil in Scutum off the 
tail of Aquila, as noted by Gary Seronik 
on page 45. Mil is universally called the 
Wild Duck Cluster, supposedly because its 
stars resemble a V-shaped skein of ducks 
migrating south for the fall. But every 
observer I've asked wonders, where are 
the ducks? The cluster is roughly round, 
and if you go looking for V shapes, you see 
them everywhere. The most common one 
people see is big, points northeast (toward 
upper left here), and embraces the bright- 
est part of the cluster inside it. 

But that's not it. 

The nickname comes from Admiral 
William H. Smyth, author of the first 
great telescopic observing guide: A Cycle 
of Celestial Objects, published in 1844. He 
called Mil "A splendid cluster of stars... 
which somewhat resembles a flight of wild 
ducks in shape." That's not much to go on. 
And his very rough sketch {below) looks 
like it was drawn from memory. 

But the drawing provides the neces- 
sary hints. Its two bright stars can only 
be meant to represent the bright pair off 
the cluster's southeastern edge. That tells 
which way to look for the two arcs in the 
cluster. And sure enough, we see a very 
flattened V of stars in this location at 
Mil's edge, and a sharper, denser V or fan 
deeper inside facing about the same way. 
Mystery solved. Go out and look. 

Mil is unusually rich for an open 
cluster, with about 3,000 stars, but its 
Hertzsprung- Russell diagram reveals it to 
be only about 220 million years old. So it's 
not some kind of sparse globular cluster. 
It's roughly 20 light-years wide and is 
located about 6,200 light-years away. ♦ 

Compared to photos of Mil, William Henry 
Smyth's "flight of wild ducks" sketch (near 
right) isn't very realistic at all. But the sketch 
does tell where to look. Sure enough, the cluster 
displays a broad, very flattened V of stars on 
the correct side and a smaller, narrower V or fan 
inside. These patterns are a little clearer visually 
than in photos. The second illustration was 
traced from a photo for emphasis. 



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• • -iW. ", V' ;■..• ..-»■• :■' • i. ■• :v. .•: .* • • : > ■■■ V' ■ ■ '. ■:» 




OBSERVING 



Exploring the Solar System 



A Last Hurrah at Uranus 

The 7 th planet yielded one final discovery for visual observers 
just before its revealing Voyager close-up. 



Three decades ago, when CCD imaging was in its 
infancy, Stephen James O'Meara determined the rotation 
period of Uranus using the time-honored combination of 
telescope and eyeball. This remarkable feat came just at 
the close of the long era when keen-eyed visual observers 
routinely recorded finer planetary details than any camera 
could capture. 

Although Uranus is roughly four times the diameter 
of Earth, it is so remote that its apparent diameter never 
exceeds 3.7 arcseconds — 13 times smaller than Jupiter 
and five times smaller than Saturn at opposition. The 
planet's low apparent surface brightness, almost 14 times 
dimmer than Jupiter's, poses an even greater handicap for 
the observer. The lack of contrast between Uranus's pallid 
disk and the dark sky background also accentuates the 
planet's limb darkening, making it difficult to confirm 
whether its image is even in focus. 

Few observers have ever managed to make out any 
markings on Uranus. Dusky belts and bright zones of 
extremely muted contrast have been reported for well 



k 




over a century, but they were usually only glimpsed as 
uncertain, fleeting impressions. Belief in the presence 
of markings on Uranus waned in 1970 after the Strato- 
scope II balloon lofted a 36-inch telescope to an altitude 
of 80,000 feet and obtained images of Uranus free from 
the effects of atmospheric turbulence. The planet proved 
to be utterly featureless in visible light, although the Stra- 
toscope telescope detected a dappling of prominent dark 
spots on the far smaller disk of Jupiter's satellite Io. 

Unable to follow the motion of any well-defined mark- 
ings, astronomers attempting to determine the velocity 
of Uranus's axial rotation employed spectrographs to 
measure the magnitude of the equal but opposite Doppler 
shifts in sunlight reflected from the planet's approach- 
ing and receding limbs. The accuracy of this technique 
is extremely sensitive to guiding errors and atmospheric 
turbulence, and with Uranus it's further complicated by 
the fact that the planet's rotational axis is inclined by a 
whopping 98° to the plane of its orbit. Uranus rolls along 
in its orbit on its side almost pole-first while making its 
84-year-long circuit of the Sun. The planet's poles are 
alternately pointed almost directly at the Earth at 42 -year 
intervals. In 1944 the planet's north pole appeared near 
the center of its disk, while in 1986 its south pole was sim- 
ilarly presented. At the intervening equinoxes the planet's 
equator faces Earth, as was the case in 1966 and 2007. 

Given these daunting challenges, it's hardly surpris- 
ing that rotation periods determined spectrographically 
were wildly discordant. Early in the 20th century a period 
of 10.8 hours was widely accepted, but 1977 brought a 
flurry of new values. Astronomers from Caltech and the 
University of Wisconsin suggested a rotation period of 
12.3 hours, almost twice the spin rate derived by a team of 
University of Texas investigators, who proposed a period 
of just under 24 hours. Robert Brown and Richard Goody 
of Harvard University's Center for Earth and Planetary 
Physics derived a period of 15.6 hours from spectrograms 
made with the Kitt Peak National Observatory's 4-meter 
reflector, which they soon revised to 16.3 hours. 

1977 was also the year that NASA launched the Voyager 
1 and Voyager 2 spacecraft on a "Grand Tour" mission to 
the outer planets that featured a Voyager 2 close encoun- 
ter with Uranus in January 1986. When the outbound 



54 September 2012 SKY & TELESCOPE 



Contributing editor Thomas Dobbins glimpsed a diffuse spot on Uranus when he 
observed the planet with Stephen O'Meara through the 36-inch Lick refractor in 19 
He's never seen another feature on Uranus before or since. 



omas Dobbins 




voyager z captures uoi 


i 


d 







Stephen O'Meara exaggerated contrasts in this drawing of Uranus 
on September 15, 1981. The motion of this bright cloud over a 
period of a week revealed a 16.4-hour rotation period. 



Voyagers flew past Saturn they returned images that 
verified the existence of the ghostly "spokes" in Saturn's 
B ring that O'Meara had reported three years earlier (S&T: 
July 2011, page 50). Bradford Smith, the leader of the 
Voyager imaging team, marveled at O'Meara's prowess as 
an observer and challenged him to try to determine the 
rotation period of Uranus. 

For months O'Meara saw only the limb darkening of a 
disappointingly bland globe through the Harvard College 
Observatory's venerable 9-inch Clark refractor, until two 
brilliant spots suddenly appeared on the night of July 22, 
1981. One of these features seemed to be stationary, mark- 
ing the position of the planet's south pole, while the other 
was located closer to the limb. Taken aback, O'Meara 
rushed out of the dome, sprinted across a catwalk, 
and ran down a spiral staircase to the office of Michael 
Rudenko, a frequent observing companion. 

"Another member of the staff, Peter Collins, was also 
there," O'Meara recalls. "I asked them to come up to the 
dome and take a look at Uranus. I didn't tell them anything 
about what I saw and merely asked what they could see. 
They both told me that they could see a pair of "stars" on 
the planet. ... In fact, Peter Collins said that Uranus looked 
like the Ring Nebula with two central stars rather than one. 
That's how bright and obvious these clouds were!" 

The spots persisted for several weeks. Their relative 
motion between July 23rd and August 28th suggested a 
rotation period of 16.0 hours, while a second series of obser- 
vations between September 8th and 15th yielded a value of 
16.4 hours. A third remarkably bright but very- short-lived 
cloud observed on August 27th and 28th indicated a 16.0- 
hour period. 



This sequence of Voyager 2 orange-light 
images records the motion of a bright, 
streaky cloud over an interval of 4.6 hours 
on January 14, 1986. The donut-shaped 
blemishes are shadows cast by motes of 
dust in the camera's optics. 



OBSERVING 



Exploring the Solar System 



When Richard Goody learned that O'Meara's visual 
observations confirmed the rotation period that he had 
determined spectrographically several years earlier, he 
implored O'Meara to publish his results. But the cautious 
Brian Marsden, Director of the Central Bureau for Astro- 
nomical Telegrams, balked at issuing an announcement. 
To convince Marsden that O'Meara's observations were 
credible and worthy of publication, Rudenko and another 
member of the observatory staff, Daniel Green, devised 
an experiment. Seven artists' depictions of Uranus that 
closely approximated the planet's tiny apparent size 
were affixed to the railing of a tall building located one 
kilometer from the refractor's dome. O'Meara, Rudenko, 
and Green took turns at the eyepiece and made sketches. 
O'Meara's were consistently the most detailed, accurately 
recording features on the order of only 0.5 arcsecond wide. 

His skepticism mollified by these results, Marsden 
announced O'Meara's visual discovery in International 
Astronomical Union Circular 3912, issued in early 1984. 
Two years later the motions of several bright clouds were 
recorded in contrast- enhanced Voyager 2 images. They 
revealed that features in the temperate latitudes (where 
two of O'Meara's three spots appeared) circle the planet 
within minutes of the periods he derived. 

The Voyager 2 images were as perplexing as they were 
gratifying. The level of contrast for the handful of features 
that could be distinguished amidst the hydrocarbon hazes 
of Uranus's cold, deep atmosphere in visible wavelengths 
was so low that it seemed utterly implausible that any 
visual observer could have seen so much as a hint of 
them. In the decades following his 1981 observations, 
O'Meara has never seen another well-defined feature on 
Uranus, even though he has studied the planet through 
telescopes as large as the 60-inch Mount Wilson reflector. 

In retrospect, Uranus seems to have been unusually 
quiescent during the 1986 Voyager 2 flyby. In 1994 the 
Hubble Space Telescope recorded a pair of bright connec- 
tive clouds that were strikingly similar to the features 
observed by O'Meara in 1981. Since 2004 several large 
ground-based telescopes equipped with adaptive optics 





Acquired by NASA's Hubble Space Telescope on August 14, 1994, these near- 
infrared images of Uranus capture a pair of bright clouds and the high-altitude 
haze that forms a bright "cap" above the planet's south pole. The resemblance to 
O'Meara's 1981 depictions of the planet is striking. 



Former Sky & Telescope editor Stephen O'Meara used this 9-inch 
Alvan Clark refractor, housed in an observatory atop the Harvard- 
Smithsonian Center for Astrophysics, to study Uranus in 1981. 

have recorded belts, zones, and periodic outbreaks of 
bright spots in near-infrared wavelengths, where these 
features display much higher contrast than in visible 
light. Uranus is proving to be far more dynamic than once 
thought. Long-term studies may reveal that the wildly 
exaggerated seasons that result from the planet's 98° axial 
tilt greatly influence its level of atmospheric activity. 

Although Uranus is not a memorable spectacle, bear in 
mind the experience of the first observer to report seeing 
"fairly definite features," British amateur T. H. Buffham. 
Using a 9-inch Newtonian reflector at magnifications of 
212x and 320x on two nights in January 1870, Buffham 
noticed "two round bright spots" on Uranus. Over the 
course of an hour on the first night he was able to discern 
that they were moving. Based on their nearly identical 
positions two nights later, he guessed that Uranus had 
rotated on its axis four times during the intervening 48 
hours and proposed a period of about 12 hours in a report 
published in The Astronomical Register three years later. 

Had Buffham made the equally plausible guess of 
three rotations, corresponding to a period of about 16 
hours, he would have beat O'Meara to the punch by more 
than a century. Buffham and O'Meara employed tele- 
scopes of the same modest aperture at similar magnifica- 
tions and saw transient bright spots moving at the same 
rate. Is this merely a remarkable coincidence, or proof that 
on rare occasions Uranus can be a rewarding target for 
telescopes of modest aperture? 

Uranus comes to opposition this month (see page 50). 
See if you can spot any features in the planet's usually 
bland atmosphere. + 



56 September 2012 SKY & TELESCOPE 




FOCUS ON 

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inch instruments. The six telescopes located at this site 
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student an intensive "hands-on" program. The public is 
invited during open houses. 

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P.O. Box 312, Plainfield, IL, USA 60544 

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OBSERVING 



Deep-Sky Wonders 



Guide Me, Cygnus 

The area around Deneb and Gamma Cygni is amazingly rich. 

















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The area around and between Deneb and Gamma Cygni includes hundreds 
of named nebulae and clusters; only the ones discussed in this article are 
labeled. Close-ups of the areas around Dolidze 9 and Sharpless 2-115 are shown 
on pages 59 and 60, respectively. Some labels have been omitted in these 
areas for clarity. See page 63 for a detailed discussion of the Gamma Cygni 
Nebula, which dominates the southwestern corner of this photo. 

September 2012 SKY & TELESCOPE 



Guide me, Cygnus, I may be alone 
Except for you. In the Summer hang above me, 
And in the Autumn face me from the west, 
And then in Winter I shall strive alone, — 
Alone until the Spring and your return. 

— Laurence Carter, Cygnus, 1921 

Cygnus, the Swan, accompanies us through most of the 
year. During cool September evenings, with the coloring 
leaves whispering in the dark, he flies high on silent pin- 
ions, gliding through the zenith on his westering flight. 

In this month's sky tour, we'll visit some of the deep- 
sky wonders sheltered in the crook between the Swan's 
tail (marked by brilliant Deneb) and his northern wing. 
Our first will be the attractively peculiar open cluster 
NCC 6910, perched just 33' north-northeast of Gamma 
(Y) Cyg n i> a l so known as Sadr. My 105-mm refractor at 
87x shows a 5' pattern that reminds me of a toy horse on 
wheels. Two bright, golden stars mark the wheels, while 
eight fainter stars of nearly equal magnitude shape the 
northwest-bound horse. A half-dozen very faint stars 
complete the cluster. I was not at all surprised to learn 
that NGC 6910 is sometimes called the Rocking Horse 
Cluster, which requires only a slightly different twist of 
the imagination to see. 

My 10-inch reflector at 68x reveals 24 bright to faint 
stars gathered in 9', many in striking star- chains. Large 
areas of nebulosity line the eastern and western sides 
of the field of view. Indeed, much of this part of the sky 
is raggedly fogged by patches of the vast Gamma Cygni 
Nebula (IC 1318), a complex of bright and dark nebulae 
that mantles nearly 4° of sky. The Gamma Cygni Nebula 
is discussed in detail in this month's Going Deep column 
on page 63. 

NGC 6910 is a very young cluster, with an age of 
about 13 million years. It holds many hot, blue-white 
stars, including the bright star at the horse's rear wheel. 
Approximately 3,700 light-years away from us, this star 
is dimmed by three magnitudes and yellowed by copious 
amounts of intervening dust grains in the plane of the 
Milky Way. 

A nice triangle of open clusters is centered 58' north- 
northeast of NGC 6910. Dolidze 9, Collinder 421, and 
Dolidze 11 share a field of view through any eyepiece that 
gives you a true field of 47' or more. 



Sue French welcomes your comments at scfrench@nycap.rr.com. 



Sue Frencr 



I i 



The northernmost cluster is Dolidze 9. Through 
my 130-mm refractor at 23x, I see three stars and a hint 
of fainter ones. The easternmost star is brightest at 7th 
magnitude, and the second-brightest star appears yellow. 
At 102x, eight stars form a Y shape with the base pointed 
west. Connecting the end of each branch of the Y with 
imaginary lines boxes the stars into an equilateral trian- 
gle with 3.4' sides. I see only four additional stars within 
the nominal 7' diameter of the cluster. The 10-inch scope 
at 68x exposes 22 stars, while the cluster and the sky to its 
north and east look nebulous. 

Collinder 421 is centered 28' west- southwest of 
Dolidze 9. It's a nice little misty spot sprinkled with sev- 
eral faint stars through the 130-mm scope at 23x. At 102x 
the cluster shows 22 moderately faint to faint stars loosely 
sprinkled over 8' of sky. Cr 421 stands out fairly well from 
the background in my 10-inch reflector at 68x, with an 
isosceles triangle of three lOth-magnitude stars embed- 
ded in the western side of a collection of a dozen fainter 
suns. At 115x about 30 stars bunch into an 8' group with 
indefinite borders. The triangle's southern stars look 
orange, and the northern one shines golden. A misty glow 
entangles the cluster and spreads to its west. 

Look 31' south-southeast of Dolidze 9 for a 9' arc of five 
stars, magnitude 7Vi to 10. All but the brightest one curve 
across the face of Dolidze 11. A few very faint stars clus- 
ter around the four-star arc through my 130-mm scope 
at 23x. At 102x the brightest star appears yellow, and a 
baker's dozen of faint stars plumps the group to about 7'. 
A close, faint star pair sits 1.8' west of the cluster's yellow 
gem. Dolidze 11 displays some nebulosity around the arc 




stars, darker sky to their east, and then brighter nebulos- 
ity beyond that. 

Estimates of the distances to Do 9, Do 11, and Cr 421 
are 2,800, 3,700, and 3,100 light-years, respectively, while 
their ages are assessed at 20 million, 400 million, and 1 
billion years. 

A 9th-magnitude star lies about halfway between and 
a little south of an imaginary line connecting Dolidze 
11 and Collinder 421. In my 10-inch reflector at 68x, this 
star sits at the northern rim of the relatively bright nebula 
DWB 87. This nebula covers about 5' with a lOth-magni- 
tude star at its west-southwestern edge and a nth-magni- 
tude star in its east-southeastern edge. 

The designation DWB comes from the 1969 catalog of 



Patchik 56 



,-;-;DWB87; 



A tight group of one nebula and three^clusters 
surrounds the six-star asterism Patchik 56. 



POSS-II / CALTECH / PALOMAR OBSERVATORY 



Selected Gems in Northern 

Object Type Mag(v) 


Cygnus 

Size 


RA 


Dec. 


NGC6910 


Open cluster 


7.4 


10' 


20 h 23.2 m 


+40° 47' 


Dolidze 9 


Open cluster 


— 


7' 


20 h 25.5 m 


+41° 54' 


Collinder 421 


Open cluster 


10.1 


8' 


20 h 23.3 m 


+41° 42' 


Dolidze 11 


Open cluster 


— 


V 


20 h 26.4 m 


+41° 25' 


DWB 87 


Emission nebula 


— 


7.8' x 4.3' 


20 h 24.6 m 


+41° 27' 


Patchik 56 


Asterism 


— 


3.3' x 2.2' 


20 h 24.9 m 


+41° 37' 


Sh 2-112 


Emission nebula 


— 


15' 


20 h 34.1 m 


+45° 39' 


Sh 2-115 


Emission nebula 


— 


30' x 20' 


20 h 34.5 m 


+46° 49' 


Berkeley 90 


Open cluster 


— 


5' 


20 h 35.2 m 


+46° 51' 



Angular sizes are from recent catalogs. Visually, an object's size is often smaller than the cataloged value 
and varies according to the aperture and magnification of the viewing instrument. Right ascension and 
declination are for equinox 2000.0. 



SkyandTelescope.com September 2012 



A 



OBSERVING 



Deep-Sky Wonders 




emission nebulae by Helene Dickel, Heinrich Wendker, 
and John Bieritz. The catalog lists 193 nebulae in the 
region surrounding Gamma Cygni. 

While observing this area with my 10-inch scope 
at 68x, I noticed a distinctive little asterism halfway 
between and a little north of an imaginary line connect- 
ing Dolidze 11 and Collinder 421. This star bunch was 
first noted by California amateur Dana Patchick and is 
known as Patchick 56. Six faint stars form a 2.4'-long 
group like a little leaping dolphin. At 213x Patchick 56 
garners four more stars that swell it to 3.2' north-south 
and 2' east-west. 

Although many other deep-sky delights inhabit this 



region, let's jump northward and twice as deep into 
space to visit the emission nebulae Sharpless 2-112 and 
Sharpless 2-115. 

Sharpless 2-112 dwells 1.3° west-northwest of Deneb. 
With a narrowband nebula filter and my 130-mm scope 
at 23x, this emission nebula is a faint teardrop dripping 
southward from a 9th-magnitude star. An O III filter 
gives a dark view but shows the nebula spreading a bit 
farther eastward. 

Sh 2-112 is easily visible through the 10-inch reflec- 
tor at 68x, and it shares the field with a large, dog-like 
asterism of stars southeast. The nebula appears larger 
with this scope, and the 9th-magnitude star adorns its 
northwestern edge. Just off the nebula's opposite side, an 
orange lOth-magnitude star decorates the view. A narrow- 
band filter reveals faint nebulosity northwest of the 9th- 
magnitude star and divided from it by a dark rift. At 115x 
the patchy nebula and dark rift show rather well without a 
filter and cover roughly 11'. The filterless view also favors 
the visibility of the faint stars enmeshed in the haze. 

Climbing 1.2° due north takes us to Sharpless 2-115. 
My 130-mm refractor at 37x exhibits an intricate tap- 
estry of light and dark. This lovely nebula engulfs two 
bright, very wide star pairs, with the brighter star of the 
northeastern pair boasting a golden hue. Fairly bright 
nebulosity lies southeast of this star and is disconnected 
from a fainter swath to the north. The nebula is also 
bright around the southwestern pair, where it is wide and 
elongated north-south. The entire nebula spans about 
27' x 15'. The open cluster Berkeley 90 rests just east 
of the golden star. At 117x it looks like a 3' patch of fog 
enveloping two moderately faint and four very faint stars. 
The cluster is not particularly obvious even in my 10 -inch 
scope at 166x. I see a 4 1 // x 3 1 //, north-south oval of 16 
stars. The three brightest are likely foreground stars, but 
the faint ones are more densely crowded than similar field 
stars around them. 

On your next clear night, let Cygnus guide you and 
reveal the some of the many riches he carries with him 
on his nightly flight across the sky. ♦ 



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Going Deep 



The Gamma Cygni Nebula 

This huge complex is both challenging and rewarding. 



SADR (GAMMA CYGNl) may have won a celestial 
lottery for one of the prime locations in the sky. It marks 
the breast of the celestial Swan and the center of the 
Northern Cross. It's also near the northernmost points of 
the Cygnus Star Cloud and the Milky Way's Great Rift. In 
addition, Sadr is surrounded by a rich tapestry of glitter- 
ing stars, dark absorption lanes, and the H II complex 
IC 1318, which sprawls across 4° of exclusive Milky Way 
property. Edward Emerson Barnard, who discovered 
IC 1318 photographically at Lick Observatory in 1892, 
described the Gamma Cygni region as "extraordinary 
from the vast amounts of nebulosity ... in tufts and 
masses and sheets of filmy light." 




The Gamma Cygni Nebula lies at the heart of the Northern Cross. The outlined 
area is shown in more detail on the page 64. 



Although Sadr appears to be directly involved with this 
nebulosity, it's actually a foreground star. Sadr is an F8 
supergiant approximately 1,800 light-years away — a huge 
distance for a star that appears so bright. But IC 1318 lies 
far behind it at a distance of 4,500 to 5,000 light-years. 

IC 1318 provides a stunning vista for imagers, with 
billowing red patches and elongated strips of nebulos- 
ity broken up by broad swaths of dust. Visually, though, 
this is a fairly difficult object due to its large size and low 
surface brightness. To guarantee success you'll need dark 
skies, low power, and appropriate filters. IC 1318 has no 
oxygen emissions, so it does not respond to an O III filter. 
A hydrogen-beta (HP) filter shows the nebulosity best, 
while a narrowband (UHC) filter that passes both H(3 and 
O III emissions yields brighter stars but less contrast in 
the nebula. 

My first look was in 1984 on an exceptional night in 
California's High Sierra. Using an 80-mm finder at 16x 
with an HP filter, I was shocked by the view — five large 
patches of irregular nebulosity filled the 4° field! A promi- 
nent fragment was easily visible northwest of Sadr along 
with two parallel ribbons to the east of this star. 

Since then I've viewed this region numerous times 
using a variety of instruments from 15x50 binoculars to 
my 18-inch Dobsonian. Smaller instruments with wider 
fields frame more of the diffuse nebulosity, and larger 
scopes reveal more dusty and wispy details. 

The individual components of IC 1318 have been 
assigned a variety of catalog designations by Beverly Lynds 
(LDN), Stewart Sharpless (Sh), and Sven Cederblad (Ced). 
Edwin Hubble and other researchers have also assigned 
various letter designations to IC 1318. For simplicity, I've 
followed the lettering scheme used by the Uranometria 
2000.0 Deep Sky Atlas (2nd edition, Chart A2). 

Let's start with IC 1318(b), a narrow wedge situated 2° 
northwest of Sadr. In my 18-inch it spans 40' x 8', with an 
8th-magnitude star near its center. The brightest portion 
spreads out northeast from this star, while the fainter 
southwest end thins and curves slightly toward the south. 
A hazy extension juts out from the north side. 

IC 1318(d) and IC 1318(e), centered 1° east of Sadr, 
form a pair of celestial wings nicknamed the "Butterfly 
Nebula." This field is an astonishing sight in my 18-inch, 
with two glowing swaths of nebulosity fractured by the 
broad dark lane LDN 889. 



SkyandTelescope.com September 2012 63 



Going Deep 



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The objects discussed in this article are labeled, but the photograph includes many more named objects (see, for instance, page 58). 
Sections of IC 1318, such as IC 1318(a), are labeled only with the letter. Gamma Cygni is the bright star south-southwest of NCC 6910. 



IC 1318(d) passes through the two 7th-magnitude stars, 
coursing southwest and northeast. North of these stars 
the nebulosity widens and appears interspersed with 
darker patches. In my 18-inch, scalloped edges and ragged 
shreds embellish the border with LDN 889. 

IC 1318(e), on the south side of the black rift, passes 
through several wide double stars as it sweeps southwest 
to northeast, extending at least 35' x 10'. Look for Barnard 
347, a dark streak near the center. 

Sh 2-108, located 1° south- southwest of Sadr, is visible 
in my 80-mm finder as a 60' x 40' oval glow. The edges 
can be clearly traced in my 18-inch, forming a kidney- 
shape outline with a darker bay on the south side harbor- 
ing a 6.4-magnitude star. 

The remaining sections are significantly fainter and 
require good conditions to identify. IC 1318(a) is an 
irregular gossamer glow, roughly 45' in size, residing 1.6° 
west of Sadr. A slightly brighter patch on the northwest 
side marks the border with the dark nebula Barnard 343. 



Finally, let's head 45' northwest of Sadr to the scat- 
tered cluster Collinder 419. This group is highlighted by 
E2666, a tight 2.7" pair of 6th- and 8th-magnitude stars. 
Three- dozen stars radiate outward in strings from the 
bright double. Cr 419 is involved with IC 1318(c), a low- 
contrast patch. The surrounding field seems patchy unfil- 
tered, with slightly darker and brighter regions. Adding 
a UHC filter confirms that Cr 419 is encased in a gauzy 
mist that opens east in a fan shape. 

At this point, take a break from straining your eyes and 
hop over to NCC 6910, a bright knot of several dozen stars 
just 40' north-northeast of Sadr. This issue's Deep-Sky Won- 
ders column (page 58) discusses this cluster and several 
more nearby. But the riches of northern Cygnus are impos- 
sible to exhaust; there are always many more challenging 
clusters and nebulae waiting to be explored. + 

Steve Gottlieb welcomes your comments and questions at 
steve_gottlieb@comcast.net. 



64 September 2012 SKY &, TELESCOPE 



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66 September 2012 SKY &, TELESCOPE 




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homemade alt-azimuth mount is built from crutches. 



68 September 2012 SKY & TELESCOPE 



Sissy Haas 

Can I interest you in a research project? 

I'm begging for volunteers from both hemispheres 
to observe a few double stars. There's no measuring 
involved, any size telescope is okay, and you don't need 
to be an experienced observer. If all goes well, we can 
answer a question that has dogged double-star observ- 
ers for 150 years. I should probably begin with a general 
introduction to double-star observing, for those new to 
this activity. But experienced observers can skip the next 
paragraph and go right to the heart of the project. 

Double stars are the jewels of the heavens, each with 
its own combination of colors, magnitude difference, 
and space between the two stars. They're perfect targets 
for hazy nights and small telescopes because they're so 
bright and crisp compared to the soft clouds of distant 
clusters and galaxies. And their fantastic colors, such as 
"slight yellow and pale garnet," "orange and smalt blue," 
and "light rose tint and dusty red" (quotes from Admiral 
W. H. Smyth), can be breathtaking. Some are line- of- sight 
coincidences called optical doubles: their stars are actually 
light-years apart and not physically connected. The others 
are physical pairs called binary stars: their members are 
usually less then 1% of a light-year apart, and are bound 
together by mutual gravity. The fainter member of a 
pair is called the companion. The apparent visual space 
between the two stars is called their separation. It's usu- 
ally measured in arcseconds ("). A 30" pair is wide, 15" is 
medium, and 3" or less is close. Most wide pairs are opti- 
cal and most close pairs are binary. 

How much aperture do you need to split a close pair 
— that is, see it's really two stars rather than one? The 
Reverend William R. Dawes answered this question for 
pairs of equally bright stars in 1859. Ever since, observ- 
ers have tried to find a formula that works just as well for 
unequal pairs. 

Let me explain all this a bit more. The Dawes formula 
says that the minimum separation splittable by a tele- 
scope is 4.56" divided by the aperture in inches, or 116" 
divided by the aperture in mm. This indicates that my 



Uneven 
Double Stars 



Your observations 
can contribute to this 
important project. 



Gamma Virginis (Porrima) 




60-mm refractor can split equal pairs 1.9" or wider, since 
116/60 = 1.9. Thus, 1.9" is known as the Dawes limit for 
my 60-mm. It doesn't mean I will always split pairs this 
tight, but it means it should be possible with good eye- 
sight and excellent optics under a good sky. 

The Dawes formula is tried and true, but it only works 
when the pair is 6th magnitude or brighter, and made of 
two stars similar in brightness. When the members are 
different by a magnitude or more, a scope won't perform 
at the Dawes limit. For example, compare Mu (|Li) Draco- 
nis with Gamma (y) Ceti, as shown on the following page. 
Their separations are almost identical. But Mu Draconis 
is a pair of twins whereas Gamma Ceti is uneven by 
nearly three magnitudes. My 60-mm scope clearly shows 
that Mu is a double star, but no sign at all that Gamma 
is one. My 12 5 -mm f/10 refractor splits Gamma about as 
easily as my 60-mm splits Mu, but I know this only from 
experience. There are no formulas as reliable as Dawes's 
for predicting it. Double-star enthusiasts have been trying 
to come up with such a formula ever since Dawes pub- 
lished his result more than a century ago. 

When I wrote a regular Sky & Telescope column on 
double stars, I received more mail about this subject than 
any other. I found six published formulas for unequal 
doubles from my own research. The only thing they 



Resolution of even pairs such as 
Gamma Viriginis (shown here) is limited 
by the size of the Airy disk, the center 
of the diffraction pattern. Double-star 
observer and graphic artist Jeremy Perez 
sketched Gamma (left to right) when it 
was slightly below the 0.76" Dawes limit 
for his 6-inch reflector, slightly above the 
limit, and comfortably above the limit. 



agree on is that pairs that are unequal by a magnitude or more 
need more separation than the Dawes limit. But none of them 
predict aperture as well as Dawes does for equal pairs, with 
the possible exception of Christopher Lord's nomogram (S&T: 
January 2002, p. 118). Lord's numbers may be right, but I know 
of no observational study that has tested them. 




Perez found 
Antares difficult 
to split in his 
6-inch scope 
due to the 
4.4-magnitude 
difference between 
its components, 
even though its 2.5" 
separation is far above 
the Dawes limit. 



SkyandTelescope.com September 2012 69 



Double Your Pleasure, Double Your Fun 



This leads us, finally, to the heart and purpose of this 
article. Why not answer the question once and for all with 
actual observations? The table on the next page shows 
representative pairs in increments of separation and 
magnitude difference. The data is taken from the online 
Washington Double Star Catalog, as of March 2011. I stuck 
with bright pairs that won't show significant changes 
within our lifetimes. Unfortunately, there isn't a pair for 
every position on the chart, and many are too far north or 
south for observers in the opposite hemisphere. Also, not 
all the pairs fit the chart perfectly. For example, Struve 
389 (E389) is actually 2.6" rather than the 2.5" that its line 
calls for. But I can only chart what actually exists, and the 
inaccuracies are too small to affect the study seriously. I 
might find pairs fainter than 7th magnitude to fill in the 
gaps and make the numbers perfect, but their extremely 
faint companion stars would place them beyond the reach 
of typical amateur telescopes. For example, even my 325- 
mm (13 -inch) Coulter Dobsonian won't show me unequal 
llth-magnitude companions, unless they're wider than 4". 

I can't compile all the observations myself, since I 
don't own a telescope of every size and can't see any 

Mu Draconis and Gamma Ceti both have separations of 2.3", 
but Gamma is a much tougher split due to the 2.6-magnitude 
difference between its components. 



pairs in the deep-southern sky. That's why I'm begging 
for volunteers to help me. If enough of you come to my 
aid, we should have a scope of the right size to test every 
position in the chart. I can always get back to you about 
missing observations that are within range of your scope 
and hemisphere. 

Now let's see how to use the chart to obtain these 
observations. Start with the first separation above the 
Dawes limit for your scope, beginning with the leftmost 
pair, whose components have a 1.0-magnitude difference. 
If you can't split that pair, you're unlikely to split any of 
the pairs to its right, which have bigger magnitude dif- 
ferences. So move down to the next line and try to split a 
wider pair with a 1.0-magnitude difference. 

If you succeed in splitting a pair, try the one to its 
right, which has the same separation but a bigger magni- 
tude difference. Keep moving to the right until you fail to 
split something or reach the rightmost column, at which 
point you should move down to the next line and start 
again on the left. 

Let me explain with an example. Let's say that you 
have a 100-mm telescope. The Dawes limit for this aper- 
ture is 1.16", so you should start with the line for 1.5", the 
first separation on the chart that's greater than 1.16". If 
you live in the Southern Hemisphere, you should begin 




Test Pairs by Separation and Magnitude Difference 



Separation 


AM =1.0 


AM =1.5 


AM = 2.0 


AM = 2.5 


AM = 3.0 


AM = 3.5 


AM = 4.0 


0.7" 


(3332 (Pup) 


1121 (Pav) 


— 


46 Vir 


— 


— 


— 


1.0" 


E2054 (Dra) 


Hrg47 (Car) 


E2403 (Dra) 


410ph 


42 Ori 


— 


— 


1.5" 


A39 (Car) 


— 


GGru 


£2303 (Set) 


— 


90 Her 


y Equ 


2.0" 


33 Ori 


JLiCyg 


49 Leo 


k Lep 


3 Mon 


— 


— 


2.5" 


CapO 16 (Cir) 


£389 (Cam) 


38Lyn 


— 


H Vel 


— 


— 


3.0" 


— 


— 


8 Boo 


VCyg 


23Aql 


yCae 


\|/ 2 Ori 


3.5" 


90 Leo 


£2671 (Cyg) 


Z1181 (Vir) 


See 180 (Cen) 


— 


h5188 (Sgr) 


— 


4.0" 


5Ser 


65 UMa 


— 


Z2958 (Peg) 


Z1878 (Dra) 


5 Aur 


E3116 (CMa) 




Detailed Data for the Test Pairs 



In addition to being a challenging split in small scopes, 
Epsilon Bootis, also known as Izar or Pulcherrima (meaning 
"most beautiful"), is famous for its contrasting colors. 

with A39 in Carina, which has a separation of 1.5" and 
a 1.0 -magnitude difference between its components. 
If you live in the U.S. you won't be able to get a good 
view of A39, so you should start with 9 Gruis, which 
is easy to spot from Florida and Texas, or X2303 in 
Scutum, which is visible everywhere except the polar 
regions. In any case, you should move to the right 
until you find a pair whose magnitude difference is 
too big for you to split. 

Then you should start on the next line, with 33 
Orionis, which again is visible anywhere outside the 
polar regions, and move left to right through that line. 

Although the logical order is top to bottom within 
the table and left to right within each line, you will 
probably end up working out of order sometimes due 
to seasonal considerations. There's no point in trying 
to split 33 Orionis in July, when it's hidden in the 
Sun's glare! If you succeed in splitting Mu Cygni, to 33 
Ori's right, you will presumably succeed on the easier 
pair 33 Ori as well. But if you fail on Mu Cygni when 
you first read this article, remember to come back and 
attempt 33 Ori when it becomes visible. 

With enough volunteers, we can finally put to rest 
the question that has dogged observers for the last cen- 
tury and a half. And that should be something which 
every participant can take pride in. This can make 
history! Please consider being part of this project; I 
can certainly use all the help I can get. Please send me 
e-mail at hasW3@comcast.net, or write to me: 

Sissy Haas 
823 Reamer Ave. 
Greensburg, PA 15601 
U.S.A. ♦ 



Longtime S&T contributor Sissy Haas is author of 
Double Stars for Small Telescopes, which is available 
from shopatsky.com. 



Const. 

Cam 

Aur 

Cae 

Lep 

Ori 

Ori 

Ori 

Mon 

CMa 

Car 

Car 

Pup 

Vel 

Lyn 

Leo 

Leo 

UMa 

Vir 

Vir 

Cen 

Dra 

Dra 

Dra 

Boo 

Cir 

Ser 

Oph 

Her 

Set 

Aql 

Pav 

Cyg 
Cyg 
Cyg 

Sgr 
Equ 
Peg 
Gru 



RA 

3 h 30.2 m 

5 h 00.3 m 

5 h 04.4 m 

5 h 13.2 m 

5 h 26.8 m 

5 h 31.2 m 

5 h 35.4 m 

6 h 01.8 m 

6 h 21.4 m 

7 h 03.3 m 

10 h 03.6 m 
7 h 279m 

8 h 56.3 m 

9 h 18.8 m 

10 h 35.0 m 

ll h 34.7 m 

ll h 55.1 m 

13 h 00.6 m 

14 h 47.1 m 

13 h 31.4 m 

14 h 42.1 m 

16 h 23.8 m 

18 h 44.3 m 

14 h 45.0 m 

15 h 29.5 m 

15 h 34.8 m 

17 h 16.6 m 

17 h 53.3 m 

18 h 20.1 m 

19 h 18.5 m 

19 h 50.7 m 

19 h 55.6 m 

20 h 18.4 m 

21 h 44.1 m 

20 h 20.5 m 

21 h 10.3 m 

22 h 56.9 m 

23 h 06.9 m 



Dec. 

+59° 22' 

+39° 24' 

-35° 29' 

-12° 56' 

+3° 06' 

+3° 18' 

-4° 50' 

-10° 36' 

-11° 46' 

-59° 11' 

-61° 53' 

-11° 33' 

-52° 43' 

+36° 48' 

+8° 39' 

+16° 48' 

+46° 29' 

-3° 22' 

+0° 58' 

-42° 28' 

+61° 16' 

+61° 42' 

+61° 03' 

+27° 04' 

-58° 21' 

+10° 32' 

-0°27' 

+40° 00' 

-7° 59' 

+1°05' 

-59° 12' 

+52° 26' 

+55° 24' 

+28° 45' 

-29° 12' 

+10° 08' 

+11° 51' 

-43° 31' 



Name 

£389 
5 Aur 
yCae 
k Lep 
\|/ 2 Ori 
33 Ori 
42 Ori 
3 Mon 
£3116 

A39 
Hrg47 

P332 

H Vel 

38 Lyn 

49 Leo 

90 Leo 

65 UMa 

46 Vir 

£1881 

See 180 

£1878 

£2054 

£2403 

8 Boo 

CapO 16 

5 Ser 
41 Oph 
90 Her 
£2303 
23 Aql 

1121 

YCyg 
£2671 

HCyg 
h5188 
y Equ 
£2958 
0Gru 



Sep. 

2.6" 
4.1" 
3.2" 
2.0" 
2.9" 
1.9" 
1.1" 
1.9" 
3.9" 
1.4" 
1.2" 
0.7" 
2.6" 
2.6" 
2.1" 
3.5" 
3.9" 
0.7" 
3.5" 
3.6" 
4.1" 
1.0" 
1.1" 
3.0" 
2.4" 
4.0" 
1.0" 
1.6" 
1.6" 
3.2" 
0.8" 
3.0" 
3.6" 
1.9" 
3.6" 
1.5" 
3.9" 
1.5" 



M n 


M 2 


6.4 


7.9 


6.0 


9.5 


4.7 


8.2 


4.4 


6.8 


4.6 


8.6 


5.7 


6.7 


4.6 


7.5 


5.0 


8.0 


5.6 


9.7 


5.8 


6.8 


6.3 


7.9 


6.2 


7.3 


4.7 


7.7 


3.9 


6.1 


5.8 


7.9 


6.3 


7.3 


6.7 


8.3 


6.2 


8.8 


6.7 


8.8 


6.7 


9.2 


6.3 


9.2 


6.1 


7.1 


6.3 


8.3 


2.6 


4.8 


7.0 


8.0 


4.2 


5.2 


4.9 


7.5 


5.3 


8.8 


6.6 


9.2 


5.3 


8.3 


5.6 


7.2 


5.0 


7.5 


6.0 


7.5 


4.7 


6.2 


6.7 


10.1 


4.7 


8.7 


6.6 


9.1 


4.5 


6.6 



AM 

1.5 
3.5 
3.5 
2.4 
4.0 
1.0 
2.9 
3.0 
4.1 
1.0 
1.6 
1.1 
3.0 
2.2 
2.1 
1.0 
1.6 
2.6 
2.1 
2.5 
2.9 
1.0 
2.0 
2.2 
1.0 
1.0 
2.6 
3.5 
2.6 
3.0 
1.6 
2.5 
1.5 
1.5 
3.4 
4.0 
2.5 
2.1 



SkyandTelescope.com September 2012 71 



^ Telescope Maintenance 



Easy Reflector 
Collimation 



Achieving precise optical alignment is neither difficult 
nor time consuming — provided you stay on course. 

Mi 

Gary Seronik 




I've been building and using telescopes for more than 
three decades, and I know from experience that collimat- 
ing a Newtonian reflector is easy. So why are many people 
intimidated by the thought of adjusting the optics in 
their telescopes? One reason may be that they've done too 
much homework, searching online and reading every- 
thing they've found. And now they're lost in a forest of 
information — some of it densely technical, and, in some 
cases, even contradictory. This is one instance when less 
really can be more. 

I see a lot of collimation guides that are heavily loaded 
with confusing details. They aren't written for the begin- 




ner, but rather for people like me — hardcore telescope 
nuts interested in the minutiae of how their instruments 
work. How do you spot collimation advice that is overly 
complicated? Here's one fool-proof way. If the first step 
says to "square the focuser," then a beginner is in for a 
rough ride. 

Collimation is a lot like routine car maintenance. 
Everyone needs to do it now and then to keep a car 
running properly, but some people will happily spend 
an entire Sunday afternoon under the hood of their car 
because they just like to tinker. If you're a telescope nut, 
collimation can be a form of tinkering. But the thing to 
remember is that it doesn't have to be. If you've recently 
acquired a reflector and are confused by the collimation 
process, then read on. 

Getting Started 

Despite a Newtonian reflector's apparent complexity, it 
has only three optical parts: the main mirror (called the 
primary), a small, flat mirror near the top of the tube 
(known as the diagonal, or secondary mirror), and the 
magnifying eyepiece. So what are we trying to do when 
we collimate a Newtonian? Just this: ensure that the 
eyepiece is aimed at the center of the primary mirror, and 
that the primary is aimed at the center of the eyepiece. 
That's all there is to it. And once you're familiar with 
the process of adjusting a scope's collimation, it rarely 
takes more than a minute or two to touch up the optical 
alignment. Indeed, on many nights you'll only have to 
confirm that everything's still okay since the last time you 
collimated the scope. 

The first thing you need is the right tool for the job. 
You can check out the article "Collimation Tools: What 



72 September 2012 SKY & TELESCOPE 





Xx ^fBLr Spi 


der 


A crucial step in the 
collimation process 


^i* \ 


JP?^^ Eyepiece 
Mb Focal plane 


is identifying the 
main telescope parts 


Secondary- 
mirror holder 


J^~~ Secondary mirror 


and familiarizing 
yourself with their 
adjustments. 

S&T DIAGRAMS 



Primary mirror 



v Collimation adjustment 
(on back of mirror cell) 



SkyandTelescope.com September 2012 73 



Telescope Maintenance 



You Need & What You Don't") on my website (www. 
garyseronik.com) for a detailed discussion, but the bottom 
line is that you really can get by with just a simple colli- 
mation cap. And if your scope didn't include one, you can 
buy a collimation cap from vendors such as Rigel Systems 
(www.rigelsys.com) for about $5. 

The second preliminary task is to ensure that the cen- 
ter of your telescope's primary mirror is marked. Many 
of today's commercial Newtonian reflectors have a small 
paper donut at the center of the primary. If yours doesn't, 



you need to mark the center (my website has instructions 
for doing this). 

Collimation is a three-step process — #1 is to roughly 
align the primary mirror; #2 is to position the secondary 
mirror; and #3 is to fine-tune the alignment of the pri- 
mary. Once the scope is collimated, on most subsequent 
nights you'll only need to perform step #3, if anything. 
It's definitely best to run through the procedure a few 
times in daylight — everything's a little trickier in the 
dark until you've gained experience. 



Step #1: Initial collimation. 



Bottom end of 
focuser draw- 
tube 



Edge of 
secondary 



Primary mirror 
"donut" 



STEP #1: 

For the sake 

of clarity, both 

the primary 

and secondary 

mirrors are 

shown far out 

of alignment in 

this diagram. 

But this is 

what you'll see 

looking into 

your reflector's 

focuser with a 

collimation cap 

inserted. Once 

you're familiar 

with the view, 

aligning the 

parts becomes 

straightforward. 




Reflection of 
collimating cap 

Secondary 
mirror holder 



Spider vanes 



Primary 
mirror clip 



Reflection of 
primary mirror 



A collimation cap is nothing more than a small plastic 
piece that slips into your telescope's focuser in place of an 
eyepiece. It has a small peep hole drilled in its center, and 
the inside is usually painted white to improve its visibility 
among the reflected images. Start the collimation process 
by putting the cap into the focuser and, looking through 
the hole in the cap, try identifying the parts shown in the 
illustration above. You should pay special attention to the 
paper donut on the primary mirror and the reflection of 
the little hole in the collimation cap that you're looking 
through, which appears as a small black dot somewhere 
on the primary's surface. The goal is to adjust the tilt of 
the primary mirror (by turning the collimation knobs on 
the mirror's cell at the rear of the scope) until that black 
dot is centered in the paper donut. 

On big scopes, it helps to have a friend make the 
adjustments while you look. Another tip is that you 
should be able to center that dot by turning just two of the 
three collimation knobs (it simplifies things if you're only 
dealing with two adjustments). I also find it easy to use 
the trial-and- error approach. Turn a knob. Does it move 
the dot closer to the center of the donut? If yes, do that 
some more. If no, turn the knob the other way, or try a 
different one. Go slowly and methodically and you should 
be able to center the dot in a minute or two. 



Step #2: Aligning the secondary mirror. 




STEP #2: When 
the primary 
mirror is correctly 
adjusted, the 
black dot of the 
collimation cap 
appears centered 
in the paper 
donut on the 
primary mirror. 



As mentioned earlier, step #1 might be all you need to 
achieve accurate collimation. But you should still check 
that your telescope's secondary mirror is correctly posi- 
tioned to ensure that the secondary is feeding 100% of the 
primary mirror's light to the eyepiece. 

Look in the collimation cap again, and this time pay 
attention to the outer circumference of the primary mir- 
ror and the outside edge of the secondary mirror. To make 
the edge of the secondary easier to see, I tape a piece of 
white paper to the inside of the telescope tube opposite 
the focuser. 

Does the primary mirror appear centered in the sec- 
ondary? That is, can you see equal amounts of blackness 
all the way around the outside edge of the main mirror? It 
might be fine — I've seen a lot of scopes arrive from the 
factory with the diagonal mirror correctly positioned. 

Most secondary mirror holders have a dizzying array 
of adjustments, and that's why aligning the mirror can be 
so intimidating. But there are really only three motions to 
work with. You can move the secondary back and forth, 
along the length of the tube. You can rotate it with respect 
to the focuser. And you can change its tilt. To some extent 
these motions are interrelated, but so long as you concen- 
trate on dealing with one at a time, you'll be fine. 

The first thing to do is get the back-and-forth distance 
correct. Use a ruler to measure the distance from the top 
of the tube to the center of the focuser. The distance from 
the top of the tube to the center of the secondary mir- 



74 September 2012 SKY &. TELESCOPE 



ror should be the same. The design of secondary-mirror 
holders varies, but usually loosening a big central nut or 
screw on the spider allows you to make the back-and-forth 
adjustment. And don't worry about getting the position 
exact — within a couple of millimeters is fine. Also, if 
you don't feel up to messing with this adjustment, leave it 
alone since chances are it was correct to begin with. 

Next, you'll want to check the rotation of the diagonal. 
Simply eyeball this by looking down the front of the 
scope — you want the face of the secondary aimed at the 
focuser. Slightly loosening and retightening the central 
nut is the usual way to adjust the mirror's rotation. 

Last, adjust the tilt of the diagonal until the outer 
edge of the primary appears concentric with the outer 
edge of the secondary when you're looking through the 
collimation cap. This often involves a small Allen key (hex 
wrench). As with the primary, if your secondary-mirror 
holder has three adjustments (and most of them do), try 
working with only two. Start with the focuser racked all 
the way in, and as you get close to good alignment, rack 
out until you see the outer edge of the primary appear to 
just about touch the edge of the secondary. 

It's important to remember that even though adjusting 
the secondary can be the most involved part of the process, 
it's also the least important. So long as you can see the 
whole primary reflected in the secondary, you'll be fine. 
Getting the primary perfectly centered up is, to some 
extent, just gravy. And you likely won't have to mess with 
this adjustment very often. I've gone years without having 
to touch the secondary mirror in some of my telescopes. It's 
just a flat piece of glass with no optical power — all it does 
is divert light from the primary mirror out the side of the 
tube. A slight misalignment isn't going to ruin the views. 



Step #3: Final adjustments. 

If you had to do a lot of fiddling with the secondary 
mirror, chances are you'll need to tweak the primary, 
though just a little. Use the same procedure as in Step #1. 
But here's a little tip: if your black dot is just a tiny bit off 
from appearing in the center of the donut on the primary 
mirror, you can make your final tweak by only adjusting 
the secondary-mirror tilt screws, which on most scopes 
you can more easily access while you're looking through 
the collimation cap in the focuser. 



STEP #3: The final 
major step in aligning 
the secondary 
mirror is to adjust 
its position so that 
the outer edge of 
the primary mirror 
appears centered in 
the secondary mirror. 




Once you've completed step #3, you're done with 
the collimation process. Go out and enjoy using your 
telescope assured that it's going to deliver the very best 
views it can. + 

Contributing editor Gary Seronik has been building and 
collimating scopes for more than three decades. He authors 
this magazine's Telescope Workshop column and can be 
contacted through his website: www.garyseronik.com. 



A Brief Collimation FAQ 



Why is collimation necessary? 

A telescope's mirror produces 
its sharpest images only at the 
center of its focal plane. So you 
want to be sure that's the part of 
the image you're magnifying with 
your eyepiece. If, instead, due to 



poor collimation, your eyepiece is 
aimed away from the center, the 
image will be blurred by an opti- 
cal aberration called coma. The 
real-world consequence is that 
you'll see less detail — Jupiter's 
subtle belts may vanish, small 



Miscollimation is obvious in out-of-focus star images seen at the 
center of the eyepiece field. The telescope producing the image at 
left is out of adjustment, while a concentric defocused star image 
at right indicates good alignment. 



lunar craters will be hard to dis- 
cern, double stars will blur into 
single points of light. Bad things. 
This is why you should collimate 
your scope — you want to ensure 
you're viewing the very best part 
of the primary's focal plane. 

What about the secondary offset? 

This is one of those collimation 
side roads that we telescope nuts 
love to head down. But for every- 
one else, if you've adjusted the 
secondary mirror so that the outer 
edge of the primary is concentric 
with it, then you've done as much 
offsetting as you need to. In fact, 
strictly speaking, the secondary 
doesn't really influence collima- 



tion — its positioning only affects 
the uniformity of the illumination 
of the focal plane. That sounds 
serious, but if you've followed the 
procedure outlined in the accom- 
panying article, you won't have to 
worry about it. 

What about laser collimators? 

I don't recommend them for 
beginners. Once you've mastered 
collimation with a simple collima- 
tion cap, you can move up to a 
tool such as the Orion Collimating 
Telescope Eyepiece from Orion 
Telescopes & Binoculars. Laser 
collimators often add a level of 
confusion for people who aren't 
familiar with the basics. 



SkyandTelescope.com September 2012 75 



Sean Walker 
Gallery 



J 




< VENUS ABOVE THE STORMY SUN 

Jim Lafferty 

During the final transit of Venus this century, 
the planet appeared to drift past a churning 
active region on the Sun's "surface" seen in the 
narrow wavelength of hydrogen- alpha light. 
Details: hunt Solar Systems LSIOOT/Ha solar tele- 
scope with an Imaging Source DMK 41AU02.AS 
video camera. Stack of multiple frames. 

► VOID IN THE VIOLET SUN 

Sebastian Voltmer 

Venus begins exiting the solar disk at the 

moment of third contact in this sharp image 

recorded in the light of calcium. 

Details: Astro-Physics 105-mmf/6 Traveler EDFS 

with Baader Hershel Safety Wedge and Calcium 

K-Line Filter, and Imaging Source DMK51AU02. 

AS video camera. Stack of multiple frames. 







1 














■ 






'_ 








• 




i JL 


.^ . :". - 


jLij 


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i 




ATRANSITING reception 

Rick Scott 

Venus and an array of communication antennas 

drift in front of the Sun in this view over the 

mountains near Chandler, Arizona. 

Details: Meade 2045 4-inch Schmidt- Cassegrain 

telescope with Thousand Oaks glass solar filter. 

Composite of two exposures, with and without the 

solar filter, combined in Adobe Photoshop. 

^SUNSET ECLIPSE 

Matt Ventimiglia 

This view of the May 21st annular eclipse was 
captured between clouds shortly before mid- 
eclipse from Meadow, Texas. 
Details: Olympus E-30 DSLR camera with 300-mm 
lens. Single snapshot captured at ISO 100. 



76 September 2012 SKY &. TELESCOPE 




a STEALING a peek 

Werner Benger 

Nearby trees and evening haze in Baton 
Rouge, Louisiana, combined to help Wer- 
ner Benger produce this beautiful photo of 
the silhouette of Venus on the solar disk. 
Details: Panasonic DMC-ZS10 digital 
camera. Snapshot recorded at ISO 100. 

^SPOTS AND SILHOUETTE 

Pete Lawrence 

As the inky-black spot of Venus transited 

the Sun, it provided observers with an 

interesting contrast to the handful of 

sunspots, showing that the spots are not 

completely black. 

Details: Vixen FL 102-S refractor with Baader 

Planetarium AstroSolar Safety Film and 

Lumenera SKYnyx2-0 video camera. Mosaic 

assembled from stacked video frames. 



SkyandTelescope.com September 2012 yy 



Gallery 




Gallery showcases the 
finest astronomical 
images submitted to us 
by our readers. Send your 
very best shots to gallery 
@SkyandTelescope. 
com. We pay $50 for each 
published photo. See 
SkyandTelescope.com/ 
aboutsky/guidelines. 



CRESCENT HAZE 

Michael P. Seiler 
Smoke from nearby 
wildfires combined 
with the diminished 
light of the eclipsed 
Sun give this scene 
above the Grand Can- 
yon an otherworldly 
appearance. 
Details: Nikon D800 
DSLR with 70-200 
mm zoom lens and 
graduated neutral- 
density filter. Compos- 
ite of two photos with 
exposures ofVso and 
V8,ooo second. ♦ 



78 September 2012 SKY &, TELESCOPE 



Visit SkyandTelescope.com/gallery for more gallery online. 



ESSENTIAL GUIDE TO ASTRONOMY 



Stais 




^TELESCOPE 



Observing Section; 
Highlights of the 
Summer Sky M3 



i 



f^s^cwpncw 



. 71* 







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EMM Inside This Issue 



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Magellanic Mystery 

Contrary to what astronomers thought for 
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Focal Point 



Dan Rinnan 



Averted Imagination 



This dismissive term may 
discourage observers from 
seeing all they can see. 



In this intensely visual hobby of ours, 
what gives me the most pleasure is finding 
faint fuzzies and occasionally seeing unex- 
pected structure in them. I enjoy arguing 
with fellow amateurs about the direction 
of spiral arms in a nearby galaxy, or how 
many stars can be counted within the 
border of the Dumbbell Nebula (M27). 

I often hear the phrase "averted imagi- 
nation" at observing sessions, a play on 
"averted vision," an effective observing 
technique of looking slightly away from a 
deep-sky object. "Averted imagination" is a 
somewhat dismissive term that bothers me. 
It's certainly a clever phrase, but I think it 
often serves to discourage observers from 
seeing as much as they can see. 

Like most enthusiastic observers, 
I have been caught "over-seeing" on a 
number of occasions, mostly when I try to 
trace out galactic or filamentary structure 
that really isn't there. That doesn't bother 
me much because I think it's a great 
exercise to push my visual perception up 
to and beyond its limits. It's gratifying 
to discover what those limits are, even 
though I occasionally have to fine-tune 
them back a few notches. 

Disciplined sketching would show that 
most of us can see more than we think we 
can. But fear of being accused of "averted 
imagination" (always with a light-hearted 
chuckle) can discourage us from making 
a try at borderline photons. At the Oregon 
Star Party a few years ago, I spoke with an 
experienced observer who regarded the 
Silver Coin Galaxy (NGC 253 in Sculp- 
tor) as a challenge. He claimed normal 
eyesight and was using an 18-inch Dobso- 
nian reflector. But this galaxy is an easy 
object even in binoculars. Perhaps he was 




intimidated by all the subtle peer pressure 
not to over- see. 

On May 5, 2012, 1 took a close but 
unaided look at the full Moon at 1:00 a.m. 
PDT. It was almost at the very moment 
of fullness, and very nearly at maximum 
perigee. The media was calling it the 
"supermoon." To me it looked considerably 
brighter than I remember (though months 
of cloudy Oregon weather may have 
dimmed my memory). And yes, it really did 
seem larger than usual. 

Like the rest of you, I roll my eyes when 
I hear of yet another internet iteration 
of the Monster Mars myth, and I'm a bit 
skeptical when someone else claims to see a 
difference between the full Moon at apogee 
and the full Moon at perigee. If I hadn't 
known of the special occasion beforehand, 
would I have noticed any difference in 
angular diameter? Probably not; almost 
certainly my imagination was at work. But 
I don't care; the sight was memorable and 



magnificent! In this case, imagination 
governs; no harm comes from being wrong. 

Einstein once wrote, "Imagination is 
more important than knowledge. Knowl- 
edge is limited. Imagination encircles the 
world." Consider for a moment these lines 
by William Blake: 

The moon, like a flower 
In heaven's high bower, 
With silver delight, 
Sits and smiles on the night. 

...and then recall what many of you 
might have heard that night: 
"Oh, wow, look at the Moon!" 
The difference is imagination. That's 
not a bad thing, though it needs to be 
checked against reality now and then. + 

Dan Rinnan has spent 55 years in his 
quest for faint fuzzies, most of it before the 
Dobsonian revolution. 



86 September 2012 SKY & TELESCOPE 



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